fix show/list/external requested_path for rewrites
fix show/list/external requested_path for rewrites

Since we're using the x-couchdb-requested-path header to track the
requested path we should expose it in the request object we pass to
the view server. Currently this object has a `requested_path` key which
has the pre-vhost path. However, when there is no virtual host it will
have the post-rewrite path. With this change it always presents the path
which was requested by the client, whether or not virtual hosts were
matched.

This should make the lives of app authors easier by giving them a
reliable source for constructing relative paths and redirects that
works with and without virtual hosts.

fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb