Fix serialization of List items if they are serialized by fields.Method. #1994
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Fixes #1993.
This PR fixes a bug that is present in marshmallow 3.15.0 and 3.16.0 at least. The problem is, list items cannot be serialized by fields.Method due to wrong schema being attached to fields.Method. This PR fixes the bug and provides a minimalistic unit test to demonstrate the bug and to test the fix.