Fix handling of fin in gun_http

Thanks David Cartt for reporting it.
Update erlang.mk
Add gun:flush/1 to easily cleanup the mailbox

It can cleanup all messages from a pid, or just the ones from a
specific stream. The latter is useful in combination with cancel/2.
Send the status as a number rather than a binary string in SPDY
Automatically calculate the content-length when possible in SPDY