New user agent added for the Freebox.

The Freebox is the multimedia device of a french Internet provider: Free. This device provides a seedbox which uses the user agent "Mozilla/5.0". In the "Content-Disposition" header, if the "filename" key is used with the "filename*=UTF-8''" value, the seedbox does not take care about the header and saves the file name with the origin URL. This patch brings the support for the Freebox users.
Unit tests added for the new Freebox user agent.
The regexp of the Freebox user agent is now more strict.

A new unit test has been added in consequence.
Merge pull request #6952 from MartialGeek/add_freebox_support

New user agent added for the Freebox.