Fix #1404 by only writing _media_fields

This just makes "date" and "original_date" into ordinary flexible fields,
which is probably the way it should be.

beets/library.py

@@ -550,7 +550,7 @@ class Item(LibModel):
         defaults to the item's path.
 
         `tags` is a dictionary of additional metadata the should be
-        written to the file.
+        written to the file. (These tags need not be in `_media_fields`.)
 
         Can raise either a `ReadError` or a `WriteError`.
         """
@@ -559,17 +559,22 @@ class Item(LibModel):
         else:
             path = normpath(path)
 
+        # Get the data to write to the file.
         item_tags = dict(self)
+        item_tags = {k: v for k, v in item_tags.items()
+                     if k in self._media_fields}  # Only write media fields.
         if tags is not None:
             item_tags.update(tags)
         plugins.send('write', item=self, path=path, tags=item_tags)
 
+        # Open the file.
         try:
             mediafile = MediaFile(syspath(path),
                                   id3v23=beets.config['id3v23'].get(bool))
         except (OSError, IOError, UnreadableFileError) as exc:
             raise ReadError(self.path, exc)
 
+        # Write the tags to the file.
         mediafile.update(item_tags)
         try:
             mediafile.save()

test/test_library.py

@@ -1058,6 +1058,14 @@ class WriteTest(unittest.TestCase, TestHelper):
         self.assertNotEqual(item.artist, 'new artist')
         self.assertEqual(MediaFile(item.path).artist, 'new artist')
 
+    def test_write_date_field(self):
+        # Since `date` is not a MediaField, this should do nothing.
+        item = self.add_item_fixture()
+        clean_year = item.year
+        item.date = 'foo'
+        item.write()
+        self.assertEqual(MediaFile(item.path).year, clean_year)
+
 
 class ItemReadTest(unittest.TestCase):