fix($http): won't parse single space response

Rails sends a single space response instead of empty headers, this
change will not attempt to parse the response

Author: gampleman

src/ng/http.js

@@ -782,7 +782,7 @@ function $HttpProvider() {
       function transformResponse(response) {
         // make a copy since the response must be cacheable
         var resp = extend({}, response);
-        if (!response.data) {
+        if (!response.data || response.data === '') {
           resp.data = response.data;
         } else {
           resp.data = transformData(response.data, response.headers, config.transformResponse);

test/ng/httpSpec.js

@@ -1133,6 +1133,17 @@ describe('$http', function() {
             expect(callback.mostRecentCall.args[0]).toEqual('');
           });
 
+          it('should not attempt to deserialize json for a blank response whose header contains application/json', function() {
+            //per http spec for Content-Type, HEAD request should return a Content-Type header
+            //set to what the content type would have been if a get was sent
+            $httpBackend.expect('GET', '/url').respond(' ', {'Content-Type': 'application/json'});
+            $http({method: 'GET', url: '/url'}).success(callback);
+            $httpBackend.flush();
+
+            expect(callback).toHaveBeenCalledOnce();
+            expect(callback.mostRecentCall.args[0]).toEqual('');
+          });
+
           it('should not deserialize tpl beginning with ng expression', function() {
             $httpBackend.expect('GET', '/url').respond('{{some}}');
             $http.get('/url').success(callback);