Depreciating X-Content-Encoded-By

as per discussion [jgen: X-Content-Encoded-By](https://groups.google.com/group/joomla-dev-general/browse_frm/thread/8327413ffe009e07/)

Before patch when setting  *Gzip Page Compression: Yes*
`X-Content-Encoded-By:Joomla! 1.6`

After patch when setting *Gzip Page Compression: Yes* + *Show Joomla! Version: Yes*
`X-Content-Encoded-By:Joomla! 3.0.3` (JVERSION)