Correctly handle nested results on transformations

[zoedsoupe]

Description
:transform directive was not handling correctly the return when a nested schema was being passed as a type, that occurs because when there's a nested data value and type, the validate_field/3 will return {:ok, <nested>} and not :ok.

given that, the :transform clause was matching only the :ok return and so on ignoring the nested data returned, so the mapper function wasn't being applied.

that didn't occur with :map type directive since the validate_field/3 clause for this type will only return :ok, accepting any map structure you're passing as value.

Related Issues
closes #12

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Checklist

• My code follows the style guidelines of this project.
• I have performed a self-review of my code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.

Additional Context
N/A