The configuration option 'cache-ttl' was used instead of 'cache-files-ttl' to determine
whether or not a cache gc should be performed.
* changed 'cache-ttl' to 'cache-files-ttl' to determine if a gc should be performed
* refactored FileDownloader to allow for easier testing
* added test to ensure that the gc is called with the proper config option
user
Nils Adermann
Filippo Tessarotto
Jordi Boggiano
Grégoire Pineau
Martin Hasoň