fix($httpBackend): set headers with falsy values

This is a breaking change. To migrate to the new behavior,
delete or set headers to `undefined` to avoid having them sent.
To restore the old behavior, override `$httpBackendProvider`
with the old implementation.

Closes #2984

src/ng/httpBackend.js

@@ -56,7 +56,9 @@ function createHttpBackend($browser, XHR, $browserDefer, callbacks, rawDocument,
       var xhr = new XHR();
       xhr.open(method, url, true);
       forEach(headers, function(value, key) {
-        if (value) xhr.setRequestHeader(key, value);
+        if (isDefined(value)) {
+            xhr.setRequestHeader(key, value);
+        }
       });
 
       // In IE6 and 7, this might be called synchronously when xhr.send below is called and the

test/ng/httpBackendSpec.js

@@ -101,6 +101,22 @@ describe('$httpBackend', function() {
     });
   });
 
+  it('should set requested headers even if they have falsy values', function() {
+    $backend('POST', 'URL', null, noop, {
+      'X-header1': 0,
+      'X-header2': '',
+      'X-header3': false,
+      'X-header4': undefined
+    });
+
+    xhr = MockXhr.$$lastInstance;
+
+    expect(xhr.$$reqHeaders).toEqual({
+      'X-header1': 0,
+      'X-header2': '',
+      'X-header3': false
+    });
+  });
 
   it('should abort request on timeout', function() {
     callback.andCallFake(function(status, response) {
@@ -388,6 +404,7 @@ describe('$httpBackend', function() {
       expect(callback).toHaveBeenCalled();
       expect(callback.mostRecentCall.args[0]).toBe(404);
     });
+
   });
 });