Adds call to ctxClear to avoid memory leak.

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