Rails Ruby on Rails

Got overhead down from 127 to 85. All tests pass:

• Tentatively replaced HeaderHash with SimpleHeaderHash, which does not preserve case but does handle converting Arrays to Strings in to_hash. This requires further discussion.
• Moved default_charset to ActionDispatch::Response to avoid having to hop over to ActionController. Ideally, this would be a constant on AD::Response, but some tests expect to be able to change it dynamically and I didn't want to change them yet.
• Completely override #initialize from Rack::Response. Previously, it was creating a HeaderHash, and then we were creating an entirely new one. There is no way to call super without incurring the overhead of creating a HeaderHash.
• Override #write from Rack::Response. Its implementation tracks Content-Length, and doing so adds additional overhead that could be mooted if other middleware changes the body. It is more efficiently done at the top-level server.
• Change sending_file to an instance_variable instead of header inspection. In general, if a state is important, it should be set as a property of the response not reconstructed later.
• Set the Etag to @body instead of .body. AS::Cache.expand_cache_key handles Arrays fine, and it's more efficient to let it handle the body parts, since it is not forced to create a joined String.
• If we detect the default cache control case, just set it, rather than setting the constituent parts and then running the normal (expensive) code to generate the string. http://github.com/rails/rails/co...