* 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.
