fix($httpBackend): don't send empty string bodies

The `XMLHttpRequest.send` spec defines different semantics for `null`
than for an empty String: an empty String should be sent with a
`Content-Type` of `text/plain`, whereas `null` should have no
`Content-Type` header set.

Closes angular#2149

src/ng/httpBackend.js

@@ -83,7 +83,7 @@ function createHttpBackend($browser, XHR, $browserDefer, callbacks, rawDocument,
         xhr.responseType = responseType;
       }
 
-      xhr.send(post || '');
+      xhr.send(post || null);
     }
 
     if (timeout > 0) {

test/ng/httpBackendSpec.js

@@ -68,6 +68,12 @@ describe('$httpBackend', function() {
     expect(xhr.$$async).toBe(true);
   });
 
+  it('should pass null to send if no body is set', function() {
+    $backend('GET', '/some-url', null, noop);
+    xhr = MockXhr.$$lastInstance;
+
+    expect(xhr.$$data).toBe(null);
+  });
 
   it('should normalize IE\'s 1223 status code into 204', function() {
     callback.andCallFake(function(status) {