Call undefine_attribute_methods only when defining new attributes

[oscardelben]

Every time we call attribute_method_prefix and related methods, we also call undefine_attribute_methods behind the scenes. I think it'd be more clear and possibly more efficient to just do it before we define new attributes.

[drogus]

I think undefine_attribute_methods should stay there, the purpose of this was probably to allow to call those methods also after define attributes was called.

[oscardelben]

The docs specifically say that define_attributes must be after attribute_method_prefix, etc. We super optimize methods definition here, so I thought it didn't make sense to call undefine_attribute_methods every time.

[drogus]

If the docs say so, I guess it's safe to remove.

/cc @spastorino @josevalim

[steveklabnik]

This no longer merges cleanly, and will need a rebase.

[oscardelben]

Will do

[oscardelben]

rebased. cc @drogus