support `default` value worked with `enum`

gadget_json_generated_form.js

@@ -111,24 +111,30 @@
 
   function render_selection(schema, json_document) {
     var input = document.createElement("select"),
-      option = document.createElement("option"),
-      option_index,
-      optionz;
+      option,
+      option_index;
     input.size = 1;
-    option.value = "";
-    if (json_document === undefined) {
-      option.selected = "selected";
+    if (schema.default) {
+      if (json_document === undefined) {
+        json_document = schema.default;
+      }
+    } else {
+      option = document.createElement("option");
+      option.value = "";
+      if (json_document === undefined) {
+        option.selected = "selected";
+      }
+      input.appendChild(option);
     }
-    input.appendChild(option);
     for (option_index in schema['enum']) {
       if (schema['enum'].hasOwnProperty(option_index)) {
-        optionz = document.createElement("option");
-        optionz.value = schema['enum'][option_index];
-        optionz.textContent = schema['enum'][option_index];
+        option = document.createElement("option");
+        option.value = schema['enum'][option_index];
+        option.textContent = schema['enum'][option_index];
         if (schema['enum'][option_index] === json_document) {
-          optionz.selected = "selected";
+          option.selected = "selected";
         }
-        input.appendChild(optionz);
+        input.appendChild(option);
       }
     }
     return input;