Adds call to ctxClear to avoid memory leak.

[ghost]

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())
    }
}

---

[justinas]

A very nice catch! Though I really wanted to do this a bit differently (a perfect use case for defer, in my opinion). I hope you don't mind. Thanks for finding the bug.

[ghost]

No problem! I'll continue playing with the code if I have time.