Don't require access_token for public gists

If you give github a blank token, then it fails where it would have succeeded given no token.

e.g.
  
  https://api.github.com/gists/9546462?access_token=

is bad, where

  https://api.github.com/gists/9546462

works.
Only supply an Authorization header if a token is passed
Use var and identity comparator !==
Merge pull request #10 from pwaller/patch-1

Don't require access_token for public gists