Use explicit type annotations for CurrentAttributes methods

[tavianator]

Motivation

When an explicit type is given for an attributes, e.g. by

attribute :user_id

#: -> Integer?
def user_id = super

we should use that type in the class methods rather than untyped.

Implementation

Rather than grab the method from the class, use instance_method() for
both the class and the GeneratedAttributesMethod signatures.

Tests

Yes.