Fixed #20348 -- Consistently handle Promise objects in model fields.

All Promise objects were passed to force_text() deep in ORM query code.
Not only does this make it difficult or impossible for developers to
prevent or alter this behaviour, but it is also wrong for non-text
fields.

This commit changes `Field.get_prep_value()` from a no-op to one that
resolved Promise objects. All subclasses now call super() method first
to ensure that they have a real value to work with.

django/db/models/fields/files.py

@@ -253,6 +253,7 @@ class FileField(Field):
 
     def get_prep_value(self, value):
         "Returns field's value prepared for saving into a database."
+        value = super(FileField, self).get_prep_value(value)
         # Need to convert File objects provided via a form to unicode for database insertion
         if value is None:
             return None