Fix a race condition when switching language in the code app

www/common/sframe-common-codemirror.js

@@ -313,6 +313,9 @@ define([
         exp.contentUpdate = function (newContent) {
             var oldDoc = canonicalize($textarea.val());
             var remoteDoc = newContent.content;
+            // setValueAndCursor triggers onLocal, even if we don't make any change to the content
+            // and it may revert other changes (metadata)
+            if (oldDoc === remoteDoc) { return; }
             exp.setValueAndCursor(oldDoc, remoteDoc);
         };