formfield_overrides doesn't work for models.DateTimeField with SplitDateTimeWidget

[Marysia Lowas-Rzechonek]

When you override the default widget used in Django Admin for models.DateTimeField using formfield_overrides with any widget that derives from SplitDateTimeWidget, the form will not pass the validation with 'Enter a valid date/time.' error message.

It seems that when updating the default formfield_overrides dict, it overrides the default values instead of merging them. At present, it seems to affect only the DateTimeField.

So, this code will trigger the error: 'Enter a valid date/time.'

formfield_overrides = {models.DateTimeField: {'widgets': widgets.AdminSplitDateTime},}

And this code will be fine:

formfield_overrides = {models.DateTimeField: {'form_class': forms.SplitDateTimeField,'widget': widgets.AdminSplitDateTime},}

The problem in django/contrib/admin/option.py around line 117.

[Iacopo Spalletti]

Tested according to the OP description (minor edit made to the description). I'm able to reproduce this on 1.9 and master. 1.8 is not affeced