fix($httpBackend): send null when post-data is undefined

IE11 (and maybe others) converts an `undefined` argument to `xhr.send()` to
string (`'undefined'`) for certain request methods (e.g. DELETE). This
causes the request to appear having a body, when it shouldn't.

Closes angular#12739

Author: gkalpak

src/ng/httpBackend.js

@@ -109,7 +109,7 @@ function createHttpBackend($browser, createXhr, $browserDefer, callbacks, rawDoc
         }
       }
 
-      xhr.send(post);
+      xhr.send(isUndefined(post) ? null : post);
     }
 
     if (timeout > 0) {

test/ng/httpBackendSpec.js

@@ -44,21 +44,23 @@ describe('$httpBackend', function() {
   });
 
   it('should pass null to send if no body is set', function() {
-    $backend('GET', '/some-url', null, noop);
+    $backend('GET', '/some-url', undefined, noop);
     xhr = MockXhr.$$lastInstance;
 
     expect(xhr.$$data).toBe(null);
   });
 
-  it('should pass the correct falsy value to send if falsy body is set (excluding NaN)', function() {
-    var values = [false, 0, "", null, undefined];
-    angular.forEach(values, function(value) {
-      $backend('GET', '/some-url', value, noop);
-      xhr = MockXhr.$$lastInstance;
+  it('should pass the correct falsy value to send if falsy body is set (excluding undefined, NaN)',
+    function() {
+      var values = [false, 0, "", null];
+      angular.forEach(values, function(value) {
+        $backend('GET', '/some-url', value, noop);
+        xhr = MockXhr.$$lastInstance;
 
-      expect(xhr.$$data).toBe(value);
-    });
-  });
+        expect(xhr.$$data).toBe(value);
+      });
+    }
+  );
 
   it('should pass NaN to send if NaN body is set', function() {
     $backend('GET', '/some-url', NaN, noop);