Support http.rb 6.0

[sferik]

This patch adds support for v6 of the http gem, while maintaining support for earlier versions going back to 2.1.0.

There were various changes to the public API in v6, including:

• Chainable methods now return an HTTP::Session instead of an HTTP::Client
• Options are now passed as keyword arguments instead of a Hash
• Malformed URIs now raise an HTTP::URI::InvalidError
• Addressable is now an optional dependency, so it needs to be explicitly required
• 1xx responses are now transparently skiped

[janko]

Thanks! I'll merge it and fix CI failures.