Explicitly load descendants patch in ActiveSupport

[KaanOzkan]

Motivation

Resolves #517

We utilize Class#descendants in tapioca. As reported in the above issue in Rails versions prior to v6.1.0, upon loading of a Rails::Engine the Class redefinition that defines descendants isn't loaded.

Implementation

Explicitly load the file using our internal safe_require method.

Tests

I tested it manually. Automated testing is likely not worth the effort in comparison to the change.