Mock : improvement on treatment of superglobals and request body

This pull request intends to improve the processing of the following variables : $_GET, $_POST, $_REQUEST and BODY when using `$f3->mock($pattern,$args,$headers,$body)`. It tries to follow the same behaviour as PHP:
* **$_GET**: corresponds to the query string contained in `$pattern`. On a `GET` request, it is merged with `$args`.
* **$_POST**: equals `$args` on a `POST` request *only*.
* **$_REQUEST**: equals to `array_merge($_GET,$_POST)` (this is what PHP does *by default*).
* **BODY**: equals `$body` if it's set, otherwise is built from `$args`. Empty on a `GET` request.