[imp #87] Suggest filename for downloaded content

There exists a special header named `Content-Disposition` that allows
for suggesting a filename to the web browser when it downloads a
resource.
It is a good idea to use it for files when the URL does not suggest any
user-friendly file name.
Merge pull request #96 from satabin/feature/set-filename-header

[imp #87] Suggest filename for downloaded content