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
Set CMP0042 policy.

This policy controlls the MACOSX_RPATH configuration.
Setting it is required to get httpp compile on Mac.
Do not link rt on Mac.

The rt library was accidentally added to the DEFAULT_LIBRARIES on Mac.
fixup! Set CMP0042 policy.
Bump version to 0.5.2