fix($httpBackend): patch for Firefox bug w/ CORS and response headers

A workaround for https://bugzilla.mozilla.org/show_bug.cgi?id=608735
In FF getAllResponseHeaders() returns null if the request is the result of CORS.

Tried to format the code so that when a FF patch is released and gains enough
traction it can easily be selected and deleted. Heavily inspired by jQuery's
patch for the same bug. This patch falls short of passing through custom headers
but covers all of the "simple response headers" in the spec at
http://www.w3.org/TR/cors/

This commit should get reverted once Firefox 21 gets out.

Closes #1468

src/ng/httpBackend.js

@@ -65,8 +65,30 @@ function createHttpBackend($browser, XHR, $browserDefer, callbacks, rawDocument,
       // always async
       xhr.onreadystatechange = function() {
         if (xhr.readyState == 4) {
+          var responseHeaders = xhr.getAllResponseHeaders();
+
+          // TODO(vojta): remove once Firefox 21 gets released.
+          // begin: workaround to overcome Firefox CORS http response headers bug
+          // https://bugzilla.mozilla.org/show_bug.cgi?id=608735
+          // Firefox already patched in nightly. Should land in Firefox 21.
+
+          // CORS "simple response headers" http://www.w3.org/TR/cors/
+          var value,
+              simpleHeaders = ["Cache-Control", "Content-Language", "Content-Type",
+                                  "Expires", "Last-Modified", "Pragma"];
+          if (!responseHeaders) {
+            responseHeaders = "";
+            forEach(simpleHeaders, function (header) {
+              var value = xhr.getResponseHeader(header);
+              if (value) {
+                  responseHeaders += header + ": " + value + "\n";
+              }
+            });
+          }
+          // end of the workaround.
+
           completeRequest(callback, status || xhr.status, xhr.response || xhr.responseText,
-                          xhr.getAllResponseHeaders());
+                          responseHeaders);
         }
       };
 

test/ng/httpBackendSpec.js

@@ -116,6 +116,9 @@ describe('$httpBackend', function() {
       };
 
       this.getAllResponseHeaders = valueFn('');
+      // for temporary Firefox CORS workaround
+      // see https://github.com/angular/angular.js/issues/1468
+      this.getResponseHeader = valueFn('');
     }
 
     callback.andCallFake(function(status, response) {