Test client response object no longer includes "headers"

[Malcolm Tredinnick]

(I bumped into this some weeks back and forgot to file the bug. Just hit it again.)

The testing docs say that the test client response has a headers attribute. This was true when HttpResponse had a headers attribute, but we nuked that last year ([6225], [6235] and [6456] are the main bits). The trivial change of fixing the documentation might not be the right solution here, though, since it turns out to be useful to be able to view the full response in testing (either debug printing or just listing out the values for comparison).

So, either

1. we add a way to get the headers dict to HttpResponse (probably least preferable, since it's better for people to use get() there),
2. add something in the test client end to provide the headers dictionary, or
3. just remove that note from the docs and you have to know the header you're after (just like in normal code, I guess).

I'm going to be lazy and make this Somebody Else's Problem. Wanted to note it down whilst I remembered.