Limit req: don't truncate key value to 255 bytes.

While the module allows to use values up to 65535 bytes as a key,
that actually never worked properly.
(official changeset: http://hg.nginx.org/nginx/rev/cda4fcb9294c)
Merge pull request #544 from chobits/limit_req_fix

Limit req: don't truncate key value to 255 bytes.