fix($http): allow empty responses to be cached

Closes #3809

Author: jankuca

src/ng/http.js

@@ -892,7 +892,7 @@ function $HttpProvider() {
 
       if (cache) {
         cachedResp = cache.get(url);
-        if (cachedResp) {
+        if (isDefined(cachedResp)) {
           if (cachedResp.then) {
             // cached request has already been sent, but there is no response yet
             cachedResp.then(removePendingReq, removePendingReq);
@@ -912,7 +912,7 @@ function $HttpProvider() {
       }
 
       // if we won't have the response in cache, send the request to the backend
-      if (!cachedResp) {
+      if (isUndefined(cachedResp)) {
         $httpBackend(config.method, url, reqData, done, reqHeaders, config.timeout,
             config.withCredentials, config.responseType);
       }

test/ng/httpSpec.js

@@ -1248,6 +1248,20 @@ describe('$http', function() {
       });
 
 
+      it('should allow the cached value to be an empty string', function () {
+        cache.put('/abc', '');
+
+        callback.andCallFake(function (response, status, headers) {
+          expect(response).toBe('');
+          expect(status).toBe(200);
+        });
+
+        $http({method: 'GET', url: '/abc', cache: cache}).success(callback);
+        $rootScope.$digest();
+        expect(callback).toHaveBeenCalled();
+      });
+
+
       it('should default to status code 200 and empty headers if cache contains a non-array element',
           inject(function($rootScope) {
             cache.put('/myurl', 'simple response');