[1.10.x] Fixed #26917 -- Fixed crash in disabled ModelChoiceFields.

Partially reverted refs #25532 to fix a regression in Django 1.10.
This reintroduces a crash for disabled forms.JSONField (refs #26949),
however, that issue is also present on Django 1.9.

Thanks Ryan Schave for the test.

Backport of a5f85d891b from master

django/forms/forms.py

@@ -373,14 +373,13 @@ class BaseForm(object):
 
     def _clean_fields(self):
         for name, field in self.fields.items():
-            if field.disabled:
-                # Initial values are supposed to be clean
-                self.cleaned_data[name] = self.initial.get(name, field.initial)
-                continue
             # value_from_datadict() gets the data from the data dictionaries.
             # Each widget type knows how to retrieve its own data, because some
             # widgets split data over several HTML fields.
-            value = field.widget.value_from_datadict(self.data, self.files, self.add_prefix(name))
+            if field.disabled:
+                value = self.initial.get(name, field.initial)
+            else:
+                value = field.widget.value_from_datadict(self.data, self.files, self.add_prefix(name))
             try:
                 if isinstance(field, FileField):
                     initial = self.initial.get(name, field.initial)