This module has been soft deprecated for a long time, but since
it was used internally it wasn't throwing deprecation warnings.
Now we can throw a deprecation warning.
Without this fix, the delegation will raise the ArgumentError:
```
% bin/test -w test/per_thread_registry_test.rb
Running 1 tests in a single process (parallelization threshold is 50)
Run options: --seed 23992
# Running:
E
Error:
PerThreadRegistryTest#test_method_missing_with_kwargs:
ArgumentError: wrong number of arguments (given 1, expected 0; required keyword: x)
/Users/kamipo/src/github.com/rails/rails/activesupport/test/per_thread_registry_test.rb:9:in `foo'
/Users/kamipo/src/github.com/rails/rails/activesupport/lib/active_support/per_thread_registry.rb:55:in `foo'
/Users/kamipo/src/github.com/rails/rails/activesupport/lib/active_support/per_thread_registry.rb:57:in `method_missing'
/Users/kamipo/src/github.com/rails/rails/activesupport/test/per_thread_registry_test.rb:13:in `test_method_missing_with_kwargs'
```
