Handle non-default kwargs in deconstruct

According to the documentation [1], non-default kwargs need to get
returned.

1: https://docs.djangoproject.com/en/dev/howto/custom-model-fields/#field-deconstruction

taggit/managers.py

@@ -92,9 +92,16 @@ class TaggableManager(RelatedField, Field):
         return manager
 
     def deconstruct(self):
+        """
+        Deconstruct the object, used with migrations.
+        """
         name, path, args, kwargs = super(TaggableManager, self).deconstruct()
+        # Remove forced kwargs.
         for kwarg in ['serialize', 'null']:
             del kwargs[kwarg]
+        # Add non-default arguments.
+        if self.through is not TaggedItem:
+            kwargs['through'] = self.through
         return name, path, args, kwargs
 
     def contribute_to_class(self, cls, name):

tests/tests.py

@@ -538,3 +538,11 @@ class TagStringParseTestCase(UnitTestCase):
         self.assertEqual(edit_string_for_tags([plain, spaces, comma]), '"com,ma", "spa ces", plain')
         self.assertEqual(edit_string_for_tags([plain, comma]), '"com,ma", plain')
         self.assertEqual(edit_string_for_tags([comma, spaces]), '"com,ma", "spa ces"')
+
+
+class DeconstructTestCase(UnitTestCase):
+    def test_deconstruct_kwargs_kept(self):
+        instance = TaggableManager(through=OfficialThroughModel)
+        name, path, args, kwargs = instance.deconstruct()
+        new_instance = TaggableManager(*args, **kwargs)
+        self.assertEqual(instance.rel.through, new_instance.rel.through)