Always set ia scanner if set_scanner is True

Archive.org now supports setting multiple values for the `scanner` field.
If set_scanner is True and a custom scanner is provided in user metadata,
append the ia scanner as a second scanner field.

internetarchive/iarequest.py

@@ -119,9 +119,12 @@ def prepare_headers(self, headers, metadata, file_metadata=None, queue_derive=Tr
         metadata = {} if metadata is None else metadata
         file_metadata = {} if file_metadata is None else file_metadata
 
-        if not metadata.get('scanner') and set_scanner is True:
+        if set_scanner is True:
             scanner = f'Internet Archive Python library {__version__}'
-            metadata['scanner'] = scanner
+            if metadata.get('scanner'):
+                metadata['scanner'] = [metadata['scanner'], scanner]
+            else:
+                metadata['scanner'] = scanner
         prepared_metadata = prepare_metadata(metadata)
         prepared_file_metadata = prepare_metadata(file_metadata)