fix issue where ElementFinder.then does not return a promise

See #1152

Author: hankduan

lib/protractor.js

@@ -744,7 +744,7 @@ var buildElementHelper = function(ptor) {
     if (this.opt_actionResult_) {
       return this.opt_actionResult_.then(fn, errorFn);
     } else {
-      return fn(this);
+      return webdriver.promise.fulfilled(fn(this));
     }
   };
 

spec/basic/elements_spec.js

@@ -305,6 +305,12 @@ describe('ElementFinder', function() {
     });
     expect(successful).toEqual(false);
   });
+
+  it('should always return a promise when calling then', function() {
+    browser.get('index.html#/form');
+    var e1 = element(by.tagName('body')).then(function(){});
+    expect(e1 instanceof protractor.promise.Promise).toBe(true);
+  });
 });
 
 describe('evaluating statements', function() {