Adds call to ctxClear to avoid memory leak.

The following test code shows that there's a memory leak across uses
of ServeHTTP.

----------------------------------------------------------------------
package main

import (
	"github.com/justinas/nosurf"
	"net/http"
	"net/http/httptest"
)

func doNothing(w http.ResponseWriter, r *http.Request) {}

func dummyGet() *http.Request {
	req, err := http.NewRequest("GET", "http://dum.my/", nil)
	if err != nil {
		panic(err)
	}
	return req
}

func main() {
	h := nosurf.New(http.HandlerFunc(doNothing))
	for {
		recorder := httptest.NewRecorder()
		h.ServeHTTP(recorder, dummyGet())
	}
}
----------------------------------------------------------------------