Improve compliance with Rack spec

[noteflakes]

Issue moved over from digital-fabric/tipi#4:

• SERVER_PORT
• rack.url_scheme
• Make InputStream buffered, implement #gets and #read
• rack.session
• rack.logger
• Check correct behaviour with both http1 and http2 (actually I think the two adapters have different standards of capitalization of HTTP headers. They should be homogenized.)
• Add some tests:
  • test basic functionality under both http1 and http2
  • test downloads
  • test uploads
  • test form submissions

Note: we'll leave rack.multipart.XXX for a later time, the same for rack.hijack.