Merge pull request #2045 from Maxgis/master

avoid  error when the  callback function not exisit

context/output.go

@@ -212,7 +212,8 @@ func (output *BeegoOutput) JSONP(data interface{}, hasIndent bool) error {
 	if callback == "" {
 		return errors.New(`"callback" parameter required`)
 	}
-	callbackContent := bytes.NewBufferString(" " + template.JSEscapeString(callback))
+	callback = template.JSEscapeString(callback)
+	callbackContent := bytes.NewBufferString(" if(window." + callback + ")" + callback)
 	callbackContent.WriteString("(")
 	callbackContent.Write(content)
 	callbackContent.WriteString(");\r\n")