HTTP::Session

[tarcieri]

Many other HTTP libraries provide a session abstraction as a way of carrying state (i.e. cookies) between requests, potentially across different origins (e.g. following redirects).

http://docs.python-requests.org/en/master/user/advanced/#session-objects

I think this is the best way to solve #302 (cc @zanker)

[janko]

I that the session objects might allow us to make HTTP::Client thread-safe (see socketry/async-io#17). Currently I would not expect that HTTP::Client caches the @connection object without looking at it; and I still assumed that it's not, even though I'm very familiar with http.rb codebase by now. But with session objects it would be obvious that the HTTP::Client is (hopefully) immutable, while HTTP::Session uses the same connection.