Populate params field in Plug.Test.conn/4 when custom params are given.

When constructing a test connection and specifying custom parameters, the connection's `params` field wouldn't populate, even after calling `fetch_params/1`. As far as I can tell, the only way to get it to populate was to test with a query string.

I don't believe the change I had to make to the `Plug.Parsers` test changes the test's semantics. I thought it better to change the call there rather than modify `fetch_params/1` to merge query string params into the already existing params (in order to artificially satisfy the test), please correct me if I'm wrong. :)