Use default_url when no file is provided or is not processing

When no file is provided, delayed_paperclip did not fall back onto default_url, and instead only did that when there was no image_processing_url.

The desired functionality when having a nil file is to use the default_url.
The desired functionality when having a processing file is to use the processing_url.

This pull request provides that feature as well as testing for both of these desired scenarios.