Force Compatibility mode for IE browsers

The problem is that IE is displaying all intranet sites in compatibility mode by default. https://developers.google.com/web-toolkit/doc/latest/DevGuideIE9

This means that even if the client is using IE9, he might see the content rendered with IE7 standards. This is problematic, because until recently we were still supporting IE7, but we are moving away from that, rewriting and simplifying our code, and thus introducing more incompatibilities with older browsers.

You can read XWIKI-8284 for certain solutions regarding changing preferences in IE, but there is also another solution provided by James P in XWIKI-8891 where we could modify the header (making the change in htmlheader.vm) and adding:

<meta http-equiv="X-UA-Compatible" content="IE=Edge"/>

http://stackoverflow.com/questions/3726357/why-does-ie9-switch-to-compatibility-mode-on-my-website

For reference, there are several mails on users mailinglist, like:
http://xwiki.markmail.org/thread/of4z477sjk2essml
http://xwiki.markmail.org/thread/5hd6s2yebtco72ul
http://xwiki.markmail.org/thread/cpqeywrlubvv3jta
http://xwiki.markmail.org/thread/4uxo6il562lms54p
from user that encounters problems when Compatibility mode is enabled.