make sure internal servlet attributes are retrieved on `env['internal.attr']` (fixes #173)
minor (internal) code cleanup
if nothing happens with the capture - no header or status set - should report as handled
- weird yet works with Tomcat as before and serves static content with Jetty right again
- make sure when asked and reported isHandled as truthy it stays (due the container)
- also do some internal cleanup - whole output swalowing should get a review ...

fixes #175
support an init param for the filter to configure whether response is handled "by default"
better to keep ENV['GEM_PATH'] updates by default - keeps Bundler is more happy ...
seems current Bundler (1.6) does not pick up `Gem.path` directly from rubygems
- re-invented **jruby.rack.env.gem_path** param instead of **jruby.rack.gem_path**