Don't set request_method cookie for GET requests

The Turbolinks script assumes a GET request if no cookie is present, so sending a cookie for GET is redundant. Turbolinks immediately deletes the cookie after reading it, so there is no risk of e.g. a PUT cookie value persisting through to the next GET request.

This makes it possible to eliminate cookies on public pages for caching via varnish, for example, as discussed in #269.