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
Nope. I just tested with `io.BytesIO` and `stream.writable() == True` even after the stream has been closed. I guess it's supposed to say if that type of stream supports writing rather than if that individual stream supports it.