fixed mutable argument bug in the source dict of SourceAttribute

src/music_kraken/utils/string_processing.py

@@ -0,0 +1,9 @@
+def unify(string: str) -> str:
+    """
+    returns an unified str, to make comparosons easy.
+    an unified string has following attributes:
+     - is lowercase
+    """
+    
+    return string.lower()
+