Monkey patch to get filename from headers

[maxime-carbonneau]

Dropbox put the filename in the headers. This patch retrieve the filename with the *_url= method

[markevans]

hi - please provide tests for adding functionality like this. also I'd rather not do it by setting @filename outside of the 'filename' method. Probably the best way would be:

1. rename filename -> name_from_url
2. in apply, use 'name' => name_from_url || name_from_response(response) where name_from_response is a new private method (which name_from_url should probably be too)