Fix data race

The argument of unmarshalInput may be the same for concurrent use if it pass as graphql "variables".
So we have to copy it before setting default values

codegen/input.gotpl

@@ -2,7 +2,10 @@
 	{{- if not .HasUnmarshal }}
 	func (ec *executionContext) unmarshalInput{{ .Name }}(ctx context.Context, obj interface{}) ({{.Type | ref}}, error) {
 		var it {{.Type | ref}}
-		var asMap = obj.(map[string]interface{})
+		asMap := map[string]interface{}{}
+		for k, v := range obj.(map[string]interface{}) {
+			asMap[k] = v
+		}
 		{{ range $field := .Fields}}
 			{{- if $field.Default}}
 				if _, present := asMap[{{$field.Name|quote}}] ; !present {