Fixed a bug in AR::Base#respond_to?

Before:

  >> ActiveRecord::Base.respond_to?(:find_by_something)
  NoMethodError: undefined method `abstract_class?' for Object:Class

After:

  >> ActiveRecord::Base.respond_to?(:find_by_something)
  => false
`scope` now raises on "dangerous" name conflicts

Similar to dangerous attribute methods, a scope name conflict is
dangerous if it conflicts with an existing class method defined within
`ActiveRecord::Base` but not its ancestors.

See also #13389.

*Godfrey Chan*, *Philippe Creux*
`enum` now raises on "dangerous" name conflicts

Dangerous name conflicts includes instance or class method conflicts
with methods defined within `ActiveRecord::Base` but not its ancestors,
as well as conflicts with methods generated by other enums on the same
class.

Fixes #13389.
Merge pull request #13450 from chancancode/ar_raise_on_conflict

Raise an error if `scope` or `enum` is about to add a conflicting method to the class

Fixed #13389