Added Filter for serving gzipped files (fixed)

Hey there,

This is a great little filter which improves the performance quite a lot. One thing which was missing, was that it doesn't also gzip all the gwt files. If a GWT project inherits from com.google.gwt.precompress.Precompress, then all its compiled files get their own .gz version, which contain the gzipped version of their output.

I've written a small filter which works great with those files. It checks if the incoming request has a .gz version, if so, it redirects to serve that version, and sets the correct encoding / content type, etc.

For example, if someone goes to example.com/Module/Module.nocache.js, they would be redirected to example.com/Module/Module.nocache.js.gz, with the proper headers set to serve it as a gzipped file.

Hope this is useful. This filter works best if called after the GWTCacheControlFilter, since for .cache files, that filter would set the cache headers before the redirect is sent by this filter.