Set cookie to track session requests and errors when sharing server b…

[twalpole]

…etween sessions. When using multiple sessions with the same app Capybara, by default, reuses the server. This means when errors are raised in a session, or requests are made from the browser (ajax, etc) we have no way of knowing which session they are associated with, and therefore errors get raised in only the first session. A setting Capybara.reuse_server was recently merged into Capybara, but that has the downside of using another port, more memory usage, etc. This PR adds another method of tracking the session by adding a cookie to the browser and using that to track the session id. Unfortunately with selenium we can't add a cookie to a domain unless the browser is already on a page on that domain (other drivers can, so it may be worth moving this functionality into a method that can be overridden). To get around that I added a query parameter to every call to #visit which is stripped out by the middleware, used to identify the session, and then set as a cookie in the browser with the response so further requests should pass it back. Another thought was to have the browser visit a page in the middleware (like identify) before every #visit to set the cookie. Any thoughts anyone reading this has would be great.

[abotalov]

Should # @api private be added for Capybara::Server::Middleware (or Capybara::Server)?

[twalpole]

Probably - I'm just not convinced I like the whole solution - would much prefer if selenium allowed us to preset a cookie before the first visit