GzipFilter will double-compress application/x-gzip content

The GzipFilter only excludes zipping mime-types of application/gzip.
If a file is of type application/x-gzip it gets double-zipped, and needs to be uncompressed twice on the client.

The following is from the Javadoc on GzipFilter:

• The content-type is in the comma separated list of mimeTypes set in the <code>mimeTypes</code> initParameter or
• if no mimeTypes are defined the content-type is not "application/gzip"

This should be extended to:

• if no mimeTypes are defined the content-type is not "application/gzip" or "application/x-gzip"

application/x-gzip is the official mime-type for compressed files.
And this is also what most clients will use when sending files to the server.