Fix SystemStackError error from Folder.contents()

[jhunschejones]

The pCloud API has a weird quirk where you can get nil or [] for a folders contents when there really are contents. To work around this, the gem has a retry mechanism to double check if empty contents are truly empty. Perhaps this should be optout-able in the future, but for now it's the default behavior because unexpected empty contents can be a big problem and hard to troubleshoot.

Unfortunately, in my initial implementation, I didn't account for the case where there really were no contents in the folder, would caused a SystemStackError error when calling contents on an empty folder 🤦🏼‍♂️.

This PR resolves fix resolves that bug by manually checking if the contents call is a retry. It also reduces test mocking to hopefully prevent future bugs in this area as well 🙏🏻