add render_boolean() and use default value from schema

gadget_json_generated_form.js

@@ -140,6 +140,26 @@
     return input;
   }
 
+  function render_boolean(schema, json_document) {
+    var input,
+      schema_for_selection = {
+        type: "boolean",
+        enum: [true, false]
+      };
+    // XXX change json_document on open is not correct @bk
+    if (json_document === "true") {
+      json_document = true;
+    }
+    if (json_document === "false") {
+      json_document = false;
+    }
+    if (getDocumentType(schema.default) === "boolean") {
+      schema_for_selection.default = schema.default;
+    }
+    input = render_selection(schema_for_selection, json_document);
+    return input;
+  }
+
   function render_textarea(json_field, default_value, data_format) {
     var input = document.createElement("textarea");
     if (default_value !== undefined) {
@@ -676,16 +696,7 @@
     }
 
     if (type === "boolean") {
-      if (default_value === "true") {
-        default_value = true;
-      }
-      if (default_value === "false") {
-        default_value = false;
-      }
-      input = render_selection({
-        type: "boolean",
-        enum: [true, false]
-      }, default_value);
+      input = render_boolean(json_field, default_value);
     }
 
     if (!input && ["string", "integer", "number"].indexOf(type) >= 0) {