Return stored enumeration instances as-is from attribute getters

[sucicfilip]

Since 3.0 Enumerations::Base inherits from String. In define_attributes_getters the when String, Symbol branch now also matches an enum instance stored as another enum's attribute (it is a String now), routing it through translate_attribute and returning a plain String instead of the enum instance.

So an attribute that stores another enumeration:

class Product < Enumerations::Base
  value :basic, name: 'Basic', active: true
end

class Price < Enumerations::Base
  value :basic_monthly, product: Product.basic
end

Price.basic_monthly.product        # => "basic" (a String, was a Product before 3.0)
Price.basic_monthly.product.active # => NoMethodError: undefined method 'active' for an instance of String

Added a when Enumerations::Base branch before the String/Symbol branch so the stored instance is returned as-is. Order matters since Base < String. Also added a regression test.