Allow blank filenames in Content-Disposition field

Attachments containing an empty string as a filename in the Content-Disposition header are skipped.

This was introduced as a side-effect of commit https://github.com/mikel/mail/commit/b1b17961dbc4adbf1205b5201495b2fa6f4afebd.

Take, for example, the following Content-Disposition header:
```
Content-Disposition: inline;
  filename=""
```

Prior to that commit the parameters in the ContentDispositionField class would return:
``` ruby
{"filename"=>"\"\""}
```
After the commit the following is returned:
``` ruby
{"filename"=>""}
```

This is correct, but the code returning the filename from the ContentDispositionField class checks on `!blank?`. Which would return true before, and now returns false.

This would result in `Message#find_attachment` not decoding the attachment because nil instead of "" is returned.

For this PR I made the `ContentDispositionField#filename` implementation match the one in `ContentTypeField#filename`.