fix($http): return responseText on IE8 for requests with responseType set

[IgorMinar]

Closes #4464
Closes #4738

[IgorMinar]

@caitp can you please review?

[caitp]

If my understanding is correct, it was possible previously for people to set responseType, and this caused us to send xhr.response (which may not have been available) to the callback rather than xhr.responseText?

Reading the code, this looks okay, and I'm not sure this should hurt any of the more evergreen browsers. If I have time, then I might set up a quick test app just to try some things out, but nothing jumps out as being super broken, for what is being addressed.

[ricochet1k]

I just hit this exact problem on IE9, and the change does fix it. It should be noted somewhere that the responseType is an XHR thing and not an Angular thing, because for example, responseType:'json' gives json responses on newer browsers, and a string on IE9.