This fixes the following warnings.
```
/rails/activemodel/test/cases/nested_error_test.rb:9: warning: method redefined; discarding old test_initialize
/rails/activemodel/test/cases/error_test.rb:29: warning: previous definition of test_initialize was here
```
Add initialize_dup to deep dup.
Move proc eval and flexible message position out to Errors,
because proc eval is needed for Errors#added? and Errors#delete
