(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 think 4096 is the default today because that's what the underlying Jetty `HttpClient` class uses, but I think if Jetty were to change this out from underneath us later, we wouldn't have any code in place which would allow us to detect this.  I'm thinking it might be better for us to make an explicit `.setRequestBufferSize` call with 4096 from our `proxy/newHttpClient` function in order to better ensure that this default is being honored.