Header field related functions are now gathered into FieldStore class.
This commit only handles request. Subsequent commit will do the same
thing for response.
This commit changes nghttpx's mruby script handling. Previously we
have 2 options to specify the mruby script file to be run on request
and on response. Now they are merged into 1 option, namely
--mruby-file. It now must return object. On request, the object's
on_req(env) method is invoked with env object. Similarly, on
response, the object's on_resp(env) method is invoked. The
specification of Env object has not changed.
Tatsuhiro Tsujikawa