Fix a bunch of pending tests by providing an introspection mode for the Response object that does up-front parsing of the headers to populate things like @etag

AC::Head now doesn't have an unfulfilled Rendering dependency, and instead works just fine standalone (which means that ConditionalGet also doesn't have a Rendering dependency)

  * Move #set_cookie and #delete_cookie inline to optimize. These optimizations should
    almost certainly be sent back upstream to Rack. The optimization involves using
    an ivar for cookies instead of indexing into the headers each time.
  * Was able to use a bare Hash for headers now that cookies have their own joining
    semantics (some code assumed that the raw cookies were an Array).
  * Cache blankness of body on body=
  * Improve expand_cache_key for Arrays of a single element (common in our case)
  * Use a simple layout condition check unless conditions are used
  * Cache visible actions
  * Lazily load the UrlRewriter
  * Make etag an ivar that is set on prepare!

  * 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.

  * convert_content_type! is handled by assign_default_content_type_and_charset!
  * set_content_length! should be handled by the endpoint server. Otherwise
    each middleware that modifies the body has to do the expensive work of
    recalculating content_length.
  * convert_language! appears to be legacy. There are no tests for this
  * convert_cookies! should be handled by the new HeaderHash in Rack
  * Use an integer for .status's internal representation to avoid needing to
    do String manipulation just to find out the status

  * Significantly simplifies setting and modifying cache control in other areas

Functional test runner finalizes response just like the integration test runner. In both runners, the @response object will now behave the same.

Some functional tests will need to be updated if they are relying on preprocessed data on the response.

Deprecate template, session, assigns, and layout accessors on response object. Instead access them through the controller instance. This mainly affects functional test assertions.

Inherit TestSession from Session::AbstractStore and add indifferent access to Session::AbstractStore.