Allow dataclasses mapped to foreign serializer

Previously, when dataclass types were present in the serializer field
mapping, they were required to map to a DataclassSerializer type,
because the serializer would always receive the `dataclass` argument.
Remove this restriction.

Fixes #46.

rest_framework_dataclasses/serializers.py

@@ -502,6 +502,10 @@ def build_dataclass_field(self, field_name: str, type_info: TypeInfo) -> Seriali
         except KeyError:
             field_class = self.serializer_dataclass_field
 
+        # allow user to map a dataclass to a regular serializer Field class
+        if not issubclass(field_class, DataclassSerializer):
+            return self.build_standard_field(field_name, type_info)
+
         field_kwargs = {'dataclass': type_info.base_type,
                         'many': type_info.is_many}
 

tests/test_field_generation.py

@@ -127,6 +127,10 @@ def test_nested(self):
         DataclassSerializer.serializer_field_mapping[refclass] = subclassed_serializer
         self.check_field(refclass, subclassed_serializer, {'dataclass': refclass, 'many': False})
 
+        # customizing the dataclass serializer by putting regular Field in the field mapping
+        DataclassSerializer.serializer_field_mapping[refclass] = fields.CharField
+        self.check_field(refclass, fields.CharField, {})
+
     def test_relational(self):
         django.setup()