fix($httpBackend): Add missing expectHEAD() method

This was documented but not implemented.

With accompanying unit test to ensure the $httpBackend.expect*()
methods exist.

src/ngMock/angular-mocks.js

@@ -1520,7 +1520,7 @@ function createHttpBackendMock($rootScope, $delegate, $browser) {
 
 
   function createShortMethods(prefix) {
-    angular.forEach(['GET', 'DELETE', 'JSONP'], function(method) {
+    angular.forEach(['GET', 'DELETE', 'JSONP', 'HEAD'], function(method) {
      $httpBackend[prefix + method] = function(url, headers) {
        return $httpBackend[prefix](method, url, undefined, headers);
      };

test/ngMock/angular-mocksSpec.js

@@ -929,6 +929,14 @@ describe('ngMock', function() {
       hb = $httpBackend;
     }));
 
+    it('should provide "expect" methods for each HTTP verb', function() {
+      expect(typeof hb.expectGET).toBe("function");
+      expect(typeof hb.expectPOST).toBe("function");
+      expect(typeof hb.expectPUT).toBe("function");
+      expect(typeof hb.expectPATCH).toBe("function");
+      expect(typeof hb.expectDELETE).toBe("function");
+      expect(typeof hb.expectHEAD).toBe("function");
+    });
 
     it('should respond with first matched definition', function() {
       hb.when('GET', '/url1').respond(200, 'content', {});