Make WP Version nr and request url part of the Request class

Move the parsing of the HTTP_USER_AGENT request header to the Request class as it is (or should be and now is) a property of the request after all.

This paves the way to evaluating 'check-for-update' requests based on WP version. I.e. only offer the update if the WP version requesting complies with the 'Requires' minimum WP version for a plugin.