Uniform application of dates

[awead]

Any ldp resource from Fedora has a repository#created and repository#lastModified property. We should be able to return that value whether it is a file, object, metadata path, or version, and use consistent method names. For example, create_date should return created from the ldp source, and modified_date should return the lastModified from the ldp source.

Returning last_modified on a file appears to be here: https://github.com/projecthydra/active_fedora/blob/master/lib/active_fedora/file.rb#L15

But that isn't doing what we think it is. You'd have to call created on the most recent version of a file to get this value,