http_cookie_ident doesn't work

[daurnimator]

Since parse_http_cookie started returning a table (commit 4bb76a3 )
session.lua/parse_session_id no longer works
and hence http_cookie_ident is broken.

A fix is to replace line 55+56 of session.lua with:

local session_cookie = parse_http_cookie(cookie).session
return session_cookie and session_cookie[1] or nil