Fix random mock spec error

[ericproulx]

Hi,

This is a fix for https://github.com/ruby-grape/grape/runs/4422845217?check_suite_focus=true

Since using let_it_be, we need to reset_global! also on before :all

[grape-bot]

	1 Message
📖	We really appreciate pull requests that demonstrate issues, even without a fix. That said, the next step is to try and fix the failing tests!

Generated by 🚫 Danger