own the usage of reflection into the RegisterType

allowing the users to only pass instances of the interfaces

modules/caddyhttp/fileserver/staticfiles.go

@@ -26,7 +26,6 @@ import (
 	"os"
 	"path"
 	"path/filepath"
-	"reflect"
 	"runtime"
 	"strconv"
 	"strings"
@@ -42,8 +41,8 @@ import (
 func init() {
 	weakrand.Seed(time.Now().UnixNano())
 
-	caddy.RegisterType("http.precompressed", []reflect.Type{
-		reflect.TypeOf((*encode.Precompressed)(nil)).Elem(),
+	caddy.RegisterType("http.precompressed", []interface{}{
+		(*encode.Precompressed)(nil),
 	})
 
 	caddy.RegisterModule(FileServer{})