Line numbers carry over in clipboard using IE 11.

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Prettify some code with <pre class="prettyprint linenums">
2. Open HTML file in IE 11. 
3. Copy code block and paste into a text editor. 
(Please include HTML, not just your source code)

What is the expected output?  What do you see instead?

All other browsers tested leave out the line numbers when copy/pasting, which 
effectively lets a user copy the unaltered code.

What version are you using?  On what browser?

- prettify-4-Mar-2013.tar.bz2 
- Internet Explorer 11.0.9600.17501

Please provide any additional information below.

This is the HTML I use that calls Prettify:

    <link href="google-code-prettify/src/prettify.css" type="text/css" rel="stylesheet" />
    <script type="text/javascript" src="google-code-prettify/src/run_prettify.js?autoload=true&amp;skin=sons-of-obsidian"></script>

Original issue reported on code.google.com by djphran...@gmail.com on 16 Jan 2015 at 7:20