Request params persist after POST request
test code:

``` nim
import jester, asyncdispatch

routes:
    get "/":
        let foo = $request.params
        echo foo
        resp """<form method=post action=/foo><input type=text name=bla><input type=submit value=.></form>"""
    post "/foo":
        let foo = $request.params
        echo foo
        resp """<p>Done. <a href="/">back</a></p>"""

runForever()
```

Steps to reproduce:
1. open `/` in the browser
2. enter something in the input field and press the button
3. click `back`
4. in the log, you will see that the GET request to `/` still gets the POST parameter of `/foo`.
