Implemented writing of numerical strings.

tests/formats/test_id3.py

@@ -28,3 +28,14 @@ def test_written_gets_recognized():
             assert tag.artist == field_content
             assert tag.album == field_content
 
+
+def test_int_converted_to_numerical_string():
+    tag = SmallTag.get("formats/test.mp3")
+
+    tag.track = 123
+
+    tag.write()
+
+    tag = SmallTag.get("formats/test.mp3")
+
+    assert tag.track == 123