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.
Merge pull request #405 from mattbrictson/no-cookie-on-get

Don't set request_method cookie for GET requests