(TK-31) Fix bug with large cookie on a proxy request

I've doubled the size of the Request buffer from its default value for the HTTP Client created on a proxy request, as it was overflowing when given a very large cookie. 
I'm thinking it would be good to also have a test which shows that the proxy does explode on a large cookie when the request-buffer-size is not configured - as a way to help validate the effect of this change.  Also, I think it would be good to somehow work the content of the cookie into the response that the test is validating so that we can validate that the cookie is being proxied correctly and not just silently dropped or truncated.