Added Const for use of CtxKeys (#1511)

* Added Const for CtxKeys

* Move CtxKey Const declarations

* Fixed tests

* fix test

caddyhttp/httpserver/mitm_test.go

@@ -7,8 +7,6 @@ import (
 	"net/http/httptest"
 	"reflect"
 	"testing"
-
-	"github.com/mholt/caddy"
 )
 
 func TestParseClientHello(t *testing.T) {
@@ -287,7 +285,7 @@ func TestHeuristicFunctionsAndHandler(t *testing.T) {
 			want := ch.interception
 			handler := &tlsHandler{
 				next: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-					got, checked = r.Context().Value(caddy.CtxKey("mitm")).(bool)
+					got, checked = r.Context().Value(MitmCtxKey).(bool)
 				}),
 				listener: newTLSListener(nil, nil),
 			}