use the correct field serializer for the Encoding field by looking up field...

use the correct field serializer for the Encoding field by looking up field type instead of hardcoded 'Byte'

asyncua/ua/ua_binary.py

@@ -277,18 +277,18 @@ def create_dataclass_serializer(dataclazz):
         in enumerate(filter(lambda f: type_is_union(f.type), data_fields))
     ]
 
-    def serialize_encoding(obj):
+    def enc_value(obj):
         enc = 0
         for name, enc_val in union_fields_encodings:
             if obj.__dict__[name] is not None:
                 enc |= enc_val
-        return Primitives.Byte.pack(enc)
+        return enc
 
     encoding_functions = [(f.name, field_serializer(f)) for f in data_fields]
 
     def serialize(obj):
         return b''.join(
-            serialize_encoding(obj) if name == 'Encoding'
+            serializer(enc_value(obj)) if name == 'Encoding'
             else serializer(obj.__dict__[name])
             for name, serializer in encoding_functions
         )