Make it possible to set value before cookie is set

Using cowboy_req:set_resp_cookie/4 set's the cookie on a special place
in the Req which is not retrievable with cowboy_req:cookie/2. This patch
additionaly sets the cookie on the Meta field effectively making it
possible to use the session before it's been set on the client.
Merge pull request #1 from bipthelin/bt-session-set-fix

Make it possible to set value before cookie is set