[FEATURE ds-improved-ajax] Finer control over customizing a request

Though `ajax()` (and `ajaxOptions()`) of `DS.RESTAdapter` are marked as
private, they have been overwritten in many applications, since there is
currently no other way to customize the request.

This feature adds new public methods to `DS.RESTAdapter`, which allow to
customize the properties of a request:

- `methodForRequest` to get the HTTP verb
- `urlForRequest` to get the URL
- `headersForRequest` to get the headers
- `dataForRequest` to get the data (query params or request body)

The `params` hash passed to those methods has all the properties with
which the corresponding `find`, `createRecord`, `findQuery`, ...  call
have been invoked: store, type, snapshot(s), id(s) and query. The
`requestType` property indicates which method is requested; the possible
values are:

- `createRecord`
- `updateRecord`
- `deleteRecord`
- `query`
- `queryRecord`
- `findRecord`
- `findAll`
- `findMany`
- `findHasMany`
- `findBelongsTo`

Performing the actual AJAX request is handled by the `_makeRequest`
method, which is similar to the existing `ajax` method: it makes the
request using `jQuery.ajax` and attaches success and failure handlers.

---

Say your API handles creation of resources via PUT, this can now be
customized as follows:

``` js
// adapters/application.js
import DS from 'ember-data';

export DS.RESTAdapter.extend({
  methodForRequest: function(params) {
    if (params.requestType === 'createRecord') {
      return "PUT";
    }

    return this._super(...arguments);
  }
});
```

FEATURES.md

@@ -23,6 +23,12 @@ entry in `config/features.json`.
   `store.findRecord()` and `store.findAll()` as described in [RFC
   99](https://github.com/emberjs/rfcs/pull/99)
 
+- `ds-improved-ajax`
+
+  This feature allows to customize how a request is formed by overwriting
+  `methodForRequest`, `urlForRequest`, `headersForRequest` and `bodyForRequest`
+  in the `DS.RESTAdapter`.
+
 - `ds-references`
 
   Adds references as described in [RFC 57](https://github.com/emberjs/rfcs/pull/57)

addon/adapters/json-api.js

@@ -4,14 +4,15 @@
 
 import Ember from 'ember';
 import RESTAdapter from "ember-data/adapters/rest";
+import isEnabled from 'ember-data/-private/features';
 
 /**
   @class JSONAPIAdapter
   @constructor
   @namespace DS
   @extends DS.RESTAdapter
 */
-export default RESTAdapter.extend({
+var JSONAPIAdapter = RESTAdapter.extend({
   defaultSerializer: '-json-api',
 
   /**
@@ -98,8 +99,12 @@ export default RESTAdapter.extend({
     @return {Promise} promise
   */
   findMany(store, type, ids, snapshots) {
-    var url = this.buildURL(type.modelName, ids, snapshots, 'findMany');
-    return this.ajax(url, 'GET', { data: { filter: { id: ids.join(',') } } });
+    if (isEnabled('ds-improved-ajax')) {
+      return this._super(...arguments);
+    } else {
+      var url = this.buildURL(type.modelName, ids, snapshots, 'findMany');
+      return this.ajax(url, 'GET', { data: { filter: { id: ids.join(',') } } });
+    }
   },
 
   /**
@@ -121,14 +126,76 @@ export default RESTAdapter.extend({
     @return {Promise} promise
   */
   updateRecord(store, type, snapshot) {
-    var data = {};
-    var serializer = store.serializerFor(type.modelName);
+    if (isEnabled('ds-improved-ajax')) {
+      return this._super(...arguments);
+    } else {
+      var data = {};
+      var serializer = store.serializerFor(type.modelName);
 
-    serializer.serializeIntoHash(data, type, snapshot, { includeId: true });
+      serializer.serializeIntoHash(data, type, snapshot, { includeId: true });
 
-    var id = snapshot.id;
-    var url = this.buildURL(type.modelName, id, snapshot, 'updateRecord');
+      var id = snapshot.id;
+      var url = this.buildURL(type.modelName, id, snapshot, 'updateRecord');
 
-    return this.ajax(url, 'PATCH', { data: data });
+      return this.ajax(url, 'PATCH', { data: data });
+    }
   }
 });
+
+if (isEnabled('ds-improved-ajax')) {
+
+  JSONAPIAdapter.reopen({
+
+    methodForRequest(params) {
+      if (params.requestType === 'updateRecord') {
+        return 'PATCH';
+      }
+
+      return this._super(...arguments);
+    },
+
+    dataForRequest(params) {
+      const { requestType, ids } = params;
+
+      if (requestType === 'findMany') {
+        return {
+          filter: { id: ids.join(',') }
+        };
+      }
+
+      if (requestType === 'updateRecord') {
+        const { store, type, snapshot } = params;
+        const data = {};
+        const serializer = store.serializerFor(type.modelName);
+
+        serializer.serializeIntoHash(data, type, snapshot, { includeId: true });
+
+        return data;
+      }
+
+      return this._super(...arguments);
+    },
+
+    headersForRequest() {
+      const headers = this._super(...arguments) || {};
+
+      headers['Accept'] = 'application/vnd.api+json';
+
+      return headers;
+    },
+
+    _requestToJQueryAjaxHash() {
+      const hash = this._super(...arguments);
+
+      if (hash.contentType) {
+        hash.contentType = 'application/vnd.api+json';
+      }
+
+      return hash;
+    }
+
+  });
+
+}
+
+export default JSONAPIAdapter;