Multiple calls to attributes shouldn't result in multiple definitions of methods for accessors, equality, etc.

[louismrose]

The following example:

class Person
  extend Attributable
  attributes :name
end

class Employee < Person
  attributes :salary
end

Defining Employee results in two calls to Attributable#add_instance_methods and hence Attributable's define_method logic is called twice (once with attributes = {name: nil} and once with attributes = {name: nil, salary: nil}).

It seems redundant to add these methods more than once.