Handle unknown media length (#406)

Some media download APIs does not support partial downloads and do not
return a 'Content-length' or 'Content-range' header and is therefore
never considered done.

This change is to consider responses where the media length is
unknown to be done.

This commit could potentially break media downloads where the API
supports partial download and only returns a 'Content-length' header
when the entire file has been served/read.

This commit fixes issue #15