增加vendor

vendor/github.com/go-pay/gopay/pkg/util/string.go

@@ -0,0 +1,18 @@
+package util
+
+import "encoding/json"
+
+func ConvertToString(v interface{}) (str string) {
+	if v == nil {
+		return NULL
+	}
+	var (
+		bs  []byte
+		err error
+	)
+	if bs, err = json.Marshal(v); err != nil {
+		return NULL
+	}
+	str = string(bs)
+	return
+}