BUG: Fix broken codemirror highlighting on file rename.

When a users uses the rename dialog in the text editor, we call
_set_mode_for_model on the returned model.  The expected model to be
returned from rename has no value for mimetype, which causes us to pass
`undefined` to `CodeMirror.findModeByMIME`, which returns `undefined`
back.  We then try access an attribute of the (undefined) return value,
causing an error.

This now properly checks whether the `mimetype` attribute is set on the
input model.

IPython/html/static/edit/js/editor.js

@@ -100,8 +100,11 b' function($,'
         
         // Find and load the highlighting mode,
         // first by mime-type, then by file extension
-        var modeinfo = CodeMirror.findModeByMIME(model.mimetype);
-        if (modeinfo.mode === "null") {
+        var modeinfo;
+        if (model.mimetype) {
+            modeinfo = CodeMirror.findModeByMIME(model.mimetype);
+        }
+        if (!modeinfo || modeinfo.mode === "null") {
             // find by mime failed, use find by ext
             var ext_idx = model.name.lastIndexOf('.');