[compress] Don't set vary header if already set

I set `res.setHeader('Vary', 'Accept, Accept-Encoding')` before compress, and unfortunately I have to reset it again afterwards.
change utils.uid() to return url-safe chars. Closes #753
Merge pull request #752 from jonathanong/patch-1

[compress] Don't set vary header if already set
append Accept-Encoding to Vary when not specified
Release 2.7.3
Typo fix in timeout middleware [req->res]
Merge pull request #754 from nfarina/patch-1

Typo fix in timeout middleware [req->res]
Doc typo fix in favicon.js middleware
Merge pull request #757 from mdp/patch-1

Doc typo fix in favicon.js middleware