Fix timeouts when using keepalive in apache bench.

Set `Connection: Keep-Alive` header in response when keepalive
connection requested. According to HTTP 1.1 it is not required, because
keepalive is the default, but ab does not comply to the standard here.
So lets set it explicitly.
Merge branch '0x7f-keepalive-fix'
Add contributors file
Add missing http codes
Bump 0.5.1