Fix AR#method_missing re-dispatching into overwritten attribute methods

This was happening when a `super` call in an overwritten attribute method was triggering a `method_missing` fallback, because attribute methods haven't been generated yet.

```ruby
class Topic < ActiveRecord::Base
  def title
    # `super` would re-invoke this method if define_attribute_methods
    # hasn't been called yet, resulting in double '!' appending
    super + '!'
  end
end
```