Fixed #18523 -- Added stream-like API to HttpResponse.

Added getvalue() to HttpResponse to return the content of the response,
along with a few other methods to partially match io.IOBase.

I only added the methods/changes mentioned by unaizalakain in https://code.djangoproject.com/ticket/18523. If you want to add the rest of the methods from `io.IOBase`, let me know, I'm happy to make it fully compatible. :D
I find `assertEqual` preferable since `asserTrue` checks `bool(expr) is True` which could pass for a result we don't want here. Well, I guess what we actually want is `assertIs(expr, True)` as described in the Python docs.