Fixes problem with connections requests keep-alive and thin

@alebsack and I discovered that the env in thin is shared across many calls in a keep-alive session. This is causing problems in the levels above. Especially if the requests are processed concurrent and two are using the same ENV. The new **thin v2** uses the `on_message_begin ` to create the request object every time a request is made (this is done by *http_parser.rb*). This pull request will create a new request object for every request that is made on persistent connections, which fixes the problem vor 1.5.x also