Error serving static files when a head request is made

[writercoder]

When a head request is made to static files served by socket.io the following error is written to the server console:

debug - served static /socket.io.js
This type of response MUST NOT have a body. Ignoring data passed to end().

This generates an annoying quantity of console output when developing using live.js (http://livejs.com/) as live.js uses head request polling to determine if static file changes have been made.

[3rd-Eden]

Marked as bug, as I don't think the new static backend takes this in to account.

[3rd-Eden]

Fix and test case landed in master.