Add test to indicate Jekyll default excludes are retained
Add test to indicate Jekyll default excludes are retained

This change adds an assertion to indicate that the default Jekyll
exclude configuration is merged into the configuration set by the
pages-gem configuration. The defaults can be seen on the [Jekyll Site]
(https://jekyllrb.com/docs/configuration/default/).

After encountering the same issue described in #589 I looked at whether
the default excludes were preserved by the pages-gem, and was confused
by the difference in the behaviour I observed vs what the code said. So
I wrote this test to indicate that the code really does work as you’d
hope it did.

github/pages-gem,github/pages-gem