Add option argument and allow bucket objects to have a prefix

[duartemvix]

This is a small change to be in convention with other fog libraries that I hope will work great with Asset Sync. Which is a great tool for synchronizing assets across multiple Cloud Storage services.

To understand the reason for this change please refer to this other fog library below where they're taking this argument on the same method. I should make a disclaimer though: I could not yet give an use to this argument inside the method as of yet. Even though, it should make it compatible with Asset Sync just immediately without any problems.

https://github.com/fog/fog-aws/blob/master/lib/fog/aws/models/storage/directories.rb#L14

You can also look though this:

https://github.com/fog/fog-azure-rm/blob/master/lib/fog/azurerm/models/storage/directories.rb#L38

[duartemvix]

The Ruby 2.7.1 test in Travis only failed because it's assessing ::Time values with 1 second precision. Reducing it to a minute precision should yield a positive result for all tests.