Adding spec coverage and fix string bug

[jpotts244]

In this PR

Link to original Issue here

• Fix bug in the create method if no snapshot_name is passed in as an argument.
  Calling present? on an empty string returns runtime error:

# Failure/Error: if snapshot_name.present?
#
#     NoMethodError:
#       undefined method `present?' for "":String
#       Did you mean?  prepend

• Added some specs to expose the bug and additional test coverage for other methods.

I have a question! I would like to write some more tests but it looks like when the Mongoid class is called in the test suite or in the dev console we are getting an uninitialized constant Db::Snapshot::Mongoid error, any thoughts on resolving this either locally or otherwise?

Thanks so much! and let me know if you have any feedback or ideas 👋