Selection.getContent issue in Chrome fix

Fixes an issue in Chrome where Selection.getContent does not always return selected text

Given:
* The inline editor is opened in Chrome
* A piece of text (text only!) is selected
* A button within a menubutton is clicked
Then:
* editor.selection.getContent({format: 'text'}) returns an empty string

This commit fixes the issue, but I am not certain rng.toString will never produce any gibberish.