fix($http): fix double-quoted date issue when encoding params

$http was wrapping dates in double quotes leading to query strings like this:
  ?date=%222014-07-07T23:00:00.000Z%22
Instead of calling JSON.stringify, this fix checks to see if a param object
has a toJSON() function and uses that for encoding, falling back to
JSON.stringify as per the previous behaviour.

Closes angular#8150 and angular#6128

Author: danbarua

src/ng/http.js

@@ -1,4 +1,4 @@
-'use strict';
+'use strict';
 
 /**
  * Parse headers into key value object
@@ -990,7 +990,12 @@ function $HttpProvider() {
 
             forEach(value, function(v) {
               if (isObject(v)) {
-                v = toJson(v);
+                if (isDefined(v.toJSON)){
+                  v = v.toJSON();
+                }
+                else {
+                  v = toJson(v);
+                }
               }
               parts.push(encodeUriQuery(key) + '=' +
                          encodeUriQuery(v));

test/ng/httpSpec.js

@@ -341,6 +341,11 @@ describe('$http', function() {
         $httpBackend.expect('GET', '/url').respond('');
         $http({url: '/url', params: {}, method: 'GET'});
       });
+
+      it('should not double quote dates', function() {
+        $httpBackend.expect('GET', '/url?date=2014-07-15T17:30:00.000Z').respond('');
+        $http({url: '/url', params: {date:new Date('2014-07-15T17:30:00.000Z')}, method: 'GET'});
+      })
     });