Consolidate attribute value loading functionality in the Attribute class (custom methods, defaults, casting)

[ronaldtse]

I noticed in the MappingRule class there are serialize and deserialize methods that used the custom_methods. But now the custom method values are handled inside XmlAdapter classes, which is very strange.

Let's consolidate value processing (custom_method, default, value type casting) in a single location.

e.g. the Attribute class.

This is purely an internal refactor and will not require spec changes (new specs required for that new location) or behavioral changes of users.

Originally posted by @ronaldtse in #45 (comment)