Fix Ruby 2.7 keyword deprecations
Why
Testing the project on Ruby 2.7 (!10 (merged)) does not help at all to check deprecations (cf. https://gitlab.com/gitlab-org/gitlab-gollum-rugged_adapter/-/jobs/1018470139), we should use kwargs explicitly.
What
Related to #3 (closed), this MR fixes Ruby 2.7 keyword deprecation warnings.
Edited by Takuya Noguchi