other rfc3229 IM values:
  - diffe - not sure how hard this will be, or if i care enough to do it.
  - gzip - can i leverage mod_deflate?
  - range - this is simple, definately want to do this first.

maybe have a way to limit this filter to files in a particular directory.

add support for other formats, like rss.

the test suite is woefully incomplete.

the default_namespace stuff is lame.  can we figure out what the default
namespace from the incoming xml document is and use that instead?

handling of Content-Type header is not complete.  we're not accounting for
the fact that it can be something like application/xml; charset=foo.

add unit tests for the Content-Type: application/xml; charset=foo case.