fix for temporary files without the original_filename attribute.

This PR fix a bug in the *TempObject*,  now one can use temporary files in conjunction with dragonfly without losing the original name of the temporary file, even if the `original_filename` attribute it's not defined.

how to reproduce the actual bug
ie:
```ruby
tmp_file = Tempfile.new(['hola', '.txt'], :encoding => 'ascii-8bit')
tmp_file.write(content)
img_uid = app.store(tmp_file)
puts img_uid
```
will output something similar to this:
*2014/03/18/11/47/41/506/file*

With this PR output should be something similar to:
*...path.../hola2843-84--0.txt*