'Source' mode inserts tabs into the contents of <pre> tags

[Jude Allred]

1. go to ckeditor.com/demo

2. enter source mode

3. set the source to:

<div>
<pre>
I am not indented,
Not one bit
</pre>
</div>

4. return to wysiwyg mode. Neither line is indented.

5. click 'source' again. Notice that "I am not indented" has been intended.

6. return to wysiwyg mode again. "I am not indented" has been indented.

Expected: Switching between 'Source' and 'Wysiwyg' mode does not alter what is displayed in WYSIWYG mode

Actual: Entering source mode can modify the contents of pre tags, and that causes changes to wysiwyg mode.

[Garry Yao]

duplicate of #6413.