handling http/2 h2c upgrade

[mischief]

hi,

i was unable to successfully get h2c to work (with curl --http2 http://..). it seems that the server module does not look for the HTTP Upgrade header, and when i tried to handle it myself the library was unhappy when i tried to attach the h1_connection.socket to a h2_connection.

i also tried to set version=2 in the server.new() options table, but this also rejects HTTP/1.1 + Upgrade: h2c.

this issue is to track handling Upgrade: h2c over plaintext http.