downloaded files should be unique

[wilsonfreitas]

rb3 works with downloaded files.
These files should be unique.
If two files have the same content, they are the same.
Currently, the downloaded files are saved with a key that is related to the argument list used to download the file.
This is misleading use to the fact that some files can have different content for the same argument list when downloaded at different time moments.
To solve it, the downloaded files should be saved with a hash code built with their content.
So all files would be downloaded.
Further, the timestamp used to download the file should be used to identify different downloads.