TestRequest doesn't reset query parameters before request

I had a test that was performing multiple xhr requests, and it started failing after upgrading to 2.3.2. Turns out that the query parameters were being concatenated such that:

xhr :post, :signin, :password => 'correct'
assert_template 'signin/index'

xhr :post, :signin, :email => 'ryan@angilly.com'
assert_template 'signin/index'

was failing on the second assertion because the action was getting password and email and rendering the next action.

This is because the query parameters are no longer in @parameters (which is what currentl gets cleared). This patch clears @env['action_controller.request.query_parameters'] on the Rack::Request object and includes a test.