Merge pull request #3 from pmahoney/master

A few minor optimizations
Use correct args in servlet2rack; fix test.
Implement more methods in mock HttpServletRequest for tests.
Don't return frozen string from environment (Ramaze modifies it; others might as well).
Reject invalid but rack-like headers HTTP_CONTENT_(LENGTH|TYPE) to appease lint.
Add HTML form to Ramaze integration example for manual testing convenience.
Use a filter to serve static files that exist or follow the chain to the rack app; refactor jetty rack handler.
Make Environment.new return a new Hash so the obj.class.new pattern used by Rails works.
Only set rack.input's encoding when Encoding is available (i.e. ruby >= 1.9).
Allow initializing Rubylet::Servlet with pre-built rack application.
Remove obsolete commented out method.
Cosmetic change to Rubylet::Servlet#to_s.
Serve from alternate context path if set.  Add static_file_filter.rb file. Use the Rack app as given rather than rebuilding from config.ru.
Write string response directly to writer rather calling #to_java_bytes and writing to the output stream.