Pass base uri to serializer when writing to file.

rdflib/graph.py

@@ -1417,7 +1417,7 @@ def serialize(
                 else:
                     os_path = location
             with open(os_path, "wb") as stream:
-                serializer.serialize(stream, encoding=encoding, **args)
+                serializer.serialize(stream, base=base, encoding=encoding, **args)
         return self
 
     def print(