fix($http): do not add trailing question

Now if you pass empty object into params you will get url with unnecessary question mark.
It may cause some issues, like this [stackoverflow question](http://stackoverflow.com/questions/19575565/angularjs-resource-url-causes-question-mark-to-appear-with-no-params).
Moreover it makes unit test less readable, because that need to add redundant question to end of some  `$httpBackend` expectations.