[fix][#33344] Wrong filename detection of Content-Disposition URL headers

The problem is that the current download system tries to read the filename from the Content-Disposition expecting that filename is quoted but that's not required. So in some sites (in this example from Github) the update fails. 

This is the expected header by the system:
`[Content-Disposition] => attachment; filename="plg_sys_mootable-1.1.1.zip"`

This is the returned header from some sites: 
`[Content-Disposition] => attachment; filename=plg_sys_mootable-1.1.1.zip` 

As you can see the filename comes without the double quotes. I replaced the file name detection to work with both systems because now it's based on the equal sign. 

I provided a demo plugin to test the issue.

Tracker issue: 
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33344