Handle alternative base classes in define_attribute_methods

[casperisfine]

Ref: d429bfb#r136670440

cc @ghiculescu @rafaelfranca

Before merging, I'd like a repro, so I can include a test case though.

[casperisfine]

Error:
AttributeMethodsTest#test_#alias_attribute_method_on_a_STI_class_is_available_on_subclasses:
NoMethodError: undefined method `text' for #<#<Class:0x00007fb640e405b8> id: nil>
    /rails/activemodel/lib/active_model/attribute_methods.rb:482:in `method_missing'
    /rails/activerecord/lib/active_record/attribute_methods.rb:491:in `method_missing'
    /rails/activerecord/test/cases/attribute_methods_test.rb:1414:in `block in <class:AttributeMethodsTest>'

Seems legit.

[ghiculescu]

@casperisfine @byroot here's a replication: https://github.com/ghiculescu/rails-50706. The readme has instructions, and ghiculescu/rails-50706@eeb1882 has the minimum steps (I think) needed to replicate.

[casperisfine]

@ghiculescu thanks, so I confirm this PR fixes your repo.

However it now fails on CI, I have to dig into it.

[ghiculescu]

Thanks @byroot <3 That's working for us in CI now.