Add missing tests for `RESTClient` and `hikari.impl.special_endpoints`

[rayakame]

I am currently working on adding tests for RESTClient functions and while doing that I noticed that there are some functions that are missing tests.

I will add them sooner or later after finishing up the 3 open PR's of mine.

[rayakame]

This could be marked as a sub issue of #47

[rayakame]

• add tests for the few not covered lines in hikari.impl.special_endpoints