browser/swagger-client.js: Throw errors for null types

Avoid 'too much recursion' errors with properties like:

  "species": {
    "type": "string",
    "enum": [
      null,
      "cat",
      "dog"
    ]
  }

The JSON Schema spec says for enums [1]:

> Elements in the array MAY be of any type, including null.

But they aren't supported by Swagger 2.0, because null isn't one of
Swagger 2's types [2,3].  This commit gives a more obvious hint about
that.  It would be nice to have clarification in the spec so we could
link to tidier docs than the GitHub issue too [2].

It would be nice if I could point to line numbers (or the chain of
keys?) to find the broken entry in the parsed spec, but it doesn't
seem like that information is available in resolveAllOf and I'm not
clear enough on the larger picture here to know where that would plug
in.

[1]: http://json-schema.org/latest/json-schema-validation.html#anchor77
[2]: OAI/OpenAPI-Specification#229 (comment)
[3]: https://github.com/swagger-api/swagger-spec/blob/master/versions/2.0.md#data-types

browser/swagger-client.js

@@ -1493,6 +1493,9 @@ Resolver.prototype.resolveAllOf = function(spec, obj, depth) {
   var name;
   for(var key in obj) {
     var item = obj[key];
+    if(item === null) {
+      throw new TypeError("Swagger 2.0 does not support null types (" + obj + ").  See https://github.com/swagger-api/swagger-spec/issues/229.")
+    }
     if(item && typeof item.allOf !== 'undefined') {
       var allOf = item.allOf;
       if(Array.isArray(allOf)) {