fix($http): fix response headers with an empty value

Empty response header values are legal (http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html). Return an empty string instead of null, which is returned when the header does not exist.

Closes #7779
You're better off having your test somewhere [here](https://github.com/jamshid/angular.js/blob/28a579e4bd37fefd98e39a5a3e0ddc4dbe0b73f2/test/ng/httpSpec.js#L582). And instead of testing the `headersGetter`, try testing something similar to the actual code the user will write, so with $httpBackend and a $http call.