test($http) - add tests for functional header values

Closes angular#5784

Author: perek

test/ng/httpSpec.js

@@ -738,6 +738,32 @@ describe('$http', function() {
         $httpBackend.flush();
       });
 
+      it('should override default headers with header function', function() {
+        $httpBackend.expect('POST', '/url', 'messageBody', function(headers) {
+          return headers['Accept'] == 'Rewritten' &&
+                 headers['Content-Type'] ==  'Rewritten';
+        }).respond('');
+
+        $http({url: '/url', method: 'POST', data: 'messageBody', headers: {
+          'Accept': 'Rewritten',
+          'Content-Type': function(){return 'Rewritten';}
+        }});
+        $httpBackend.flush();
+      });
+
+      it('should remove default headers with header function', function() {
+        $httpBackend.expect('POST', '/url', 'messageBody', function(headers) {
+          return headers['Accept'] == 'Rewritten' &&
+                 typeof headers['Content-Type'] === "undefined";
+        }).respond('');
+
+        $http({url: '/url', method: 'POST', data: 'messageBody', headers: {
+          'Accept': 'Rewritten',
+          'Content-Type': function(){return null;}
+        }});
+        $httpBackend.flush();
+      });
+
       it('should override default headers with custom in a case insensitive manner', function() {
         $httpBackend.expect('POST', '/url', 'messageBody', function(headers) {
           return headers['accept'] == 'Rewritten' &&