Job#fetch_url supports URLs with non-english characters

## Before

Job#fetch_url was blowing up with a URI::InvalidURIError exception on URLs with non-english chars such as `http://static.bootic.net/167/original/69806-diseños-portada.jpg`

## Fix

Dragonfly::Job::FetchUrl makes sure to `URI.escape` its URL before parsing it and opening it, so non-english URLs work.