Make HTTP response headers available via client

[Posnet]

I've found that I need to inspect the response headers from the http upgrade request.
This pull request copies the headers from the parsed response into the client.
I realize this is a niche use case and may not make sense for inclusion. But I'd thought I'd check anyway.

[niklasad1]

Hey, thanks for your PR

I found it a bit surprising to add the response headers to the Client itself, can you explain why you didn't add it ServerResponse instead?

Otherwise, looks good to me