feat($http): make the transform defaults to an array

$httpProvider.defaults.transformRequest and $httpProvider.defaults.transformResponse
are now arrays containing single function. This makes it easy to add an
extra transform fn.

adding an extra fn before had to be done in this cluncky way:

$httpProvider.defaults.transformResponse =
[$httpProvider.defaults.transformResponse, myTransformFn];

after this change, it's simply:

$httpProvider.defaults.transformResponse.push(myTransformFn);

src/ng/http.js

@@ -90,20 +90,20 @@ function $HttpProvider() {
 
   var $config = this.defaults = {
     // transform incoming response data
-    transformResponse: function(data) {
+    transformResponse: [function(data) {
       if (isString(data)) {
         // strip json vulnerability protection prefix
         data = data.replace(PROTECTION_PREFIX, '');
         if (JSON_START.test(data) && JSON_END.test(data))
           data = fromJson(data, true);
       }
       return data;
-    },
+    }],
 
     // transform outgoing request data
-    transformRequest: function(d) {
+    transformRequest: [function(d) {
       return isObject(d) && !isFile(d) ? toJson(d) : d;
-    },
+    }],
 
     // default headers
     headers: {