fix($httpBackend): prevent DOM err due to dereferencing .responseText

If responseType is defined and the request fails for one reason or another
the .response property returned falsy value which caused dereferencing of
.responseText. If the responseType was a blob or document then an error
was thrown.

To prevent this, I'm checking for responseType first and based on that
dereferencing .response or .responseText.

We need to keep on checking .responseText because that's the original XHR
response api that is still needed for IE8 and 9.

Closes #1922