Fix dumping of custom type that mixes adapter interface

[andrykonchin]

Fix the following case:

• declared a field with custom type and adapter class
• assigned a value of class that implements #dynamoid_dump method (like a simple custom type)

A proper behaviour is to use adapter's .dynamoid_dump method instead of a value's #dynamoid_dump method.

[github-actions]

Package	Line Rate	Health
dynamoid	92%	➖
Summary	92% (3422 / 3735)	➖

Minimum allowed line rate is 90%