Enable Rubocop Rule to Freeze Constants

[mitchellhenke]

🛠 Summary of changes

Testing the waters on this a bit. Ruby constants warn when re-assigned, but are still mutable. Short illustration of the potential issue:

irb(local):001:0> C = {}
{}
irb(local):002:0> C[:a] = 1
1
irb(local):003:0> C
{:a=>1}

This PR enables the Style/MutableConstant Rubocop rule in it's default configuration. We could experimentally enable it in strict mode to try to freeze non-literal constants.

[zachmargolis]

have taken advantage of testing things in the Rails conosle by modifing IdentityConfig.store.foo = :bar, I'm sure I could work around it but I would skip this if given the option

[mitchellhenke]

Ah, in development? We could only enable in test/prod as an alternative too.

[zachmargolis]

😬 no in staging, to test API keys, etc

[mitchellhenke]

In testing this, that functionality will still work since it depends on the struct itself being frozen, which this doesn't do.

[mitchellhenke]

These tests are difficult to refactor as frozen constants, though it is in theory possible. The tests are mostly tests covering the behavior already tested in spec/lib/ab_test_bucket_spec.rb.

[mitchellhenke]

Similar to https://github.com/18F/identity-idp/pull/10340/files#r1550503287, this test group is mostly testing the bucket class rather than the implementation of it in DocAuthRouter. The test above for the nil discriminator is unique to the DocAuthRouter and won't be removed.