Fix makemigrations warning for user model case

Fix issue where a warning is raised when running makemigrations
when user model is passed with different case to the default.
This commonly happens when the default is `auth.User` and
`auth.user` is passed in the `to` field.

The fix is to detect the case difference, and update the incoming
args to match the default.

Relates to zsoldosp#43.

django_currentuser/db/models/fields.py

@@ -17,6 +17,14 @@ class CurrentUserField(models.ForeignKey):
 
     def __init__(self, *args, **kwargs):
         self.on_update = kwargs.pop("on_update", False)
+
+        # If `to` is present in kwargs, and the same when ignoring case then
+        # update `to` to use the defaults.
+        # Fix for https://github.com/PaesslerAG/django-currentuser/issues/43
+        if "to" in kwargs \
+                and kwargs["to"].lower() == self.defaults['to'].lower():
+            kwargs["to"] = self.defaults['to']
+
         self._warn_for_shadowing_args(*args, **kwargs)
 
         if "on_delete" not in kwargs:

tests/testapp/tests.py

@@ -106,6 +106,16 @@ def test_raises_warning_when_non_default_arguments_are_passed(self):
             assert_that([str(m.message) for m in my_warnings],
                         is_([CurrentUserField.warning] * 4))
 
+    def test_no_warning_raised_when_upper_case_user_model_passed(self):
+        with warnings.catch_warnings(record=True) as my_warnings:
+            self.field_cls(to='auth.User')
+            assert_that(my_warnings, has_length(0))
+
+    def test_no_warning_raised_when_lower_case_user_model_passed(self):
+        with warnings.catch_warnings(record=True) as my_warnings:
+            self.field_cls(to='auth.user')
+            assert_that(my_warnings, has_length(0))
+
     def test_no_warning_raised_if_passed_argument_values_match_defaults(self):
         with warnings.catch_warnings(record=True) as my_warnings:
             self.field_cls(default=get_current_authenticated_user)