http: speed up checkIsHttpToken

The Regex implementation is not faster than ascii code compare.

the field name is shorter, the speed is faster.

benchmark result here:

https://bitbucket.org/snippets/JacksonTian/Rnbad/benchmark-result

PR-URL: https://github.com/nodejs/node/pull/4790


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>