Renaming a model field that has null=False makes it nullable in MySQL

[murfi]

Steps to reproduce:

1. Create a simple model:

   class TestModel(models.Model):
           value_a = models.IntegerField(null=False)
   

2. ./manage.py makemigrations
3. ./manage.py migrate
4. So far so good:

   mysql> describe testy_testmodel;
   +---------+---------+------+-----+---------+----------------+
   | Field   | Type    | Null | Key | Default | Extra          |
   +---------+---------+------+-----+---------+----------------+
   | id      | int(11) | NO   | PRI | NULL    | auto_increment |
   | value_a | int(11) | NO   |     | NULL    |                |
   +---------+---------+------+-----+---------+----------------+
   2 rows in set (0.00 sec)
   

5. Change model field name value_a -> value_b:

   class TestModel(models.model):
           value_b = models.IntegerField(null=False)
   

6. ./manage.py makemigrations
7. ./manage.py migrate
8. NOT NULL for value_b has bee lost:

   mysql> describe testy_testmodel;
   +---------+---------+------+-----+---------+----------------+
   | Field   | Type    | Null | Key | Default | Extra          |
   +---------+---------+------+-----+---------+----------------+
   | id      | int(11) | NO   | PRI | NULL    | auto_increment |
   | value_b | int(11) | YES  |     | NULL    |                |
   +---------+---------+------+-----+---------+----------------+
   2 rows in set (0.00 sec)
   

The ALTER-statement django produces is

ALTER TABLE `testy_testmodel` CHANGE `value_a` `value_b` integer;

...and it should contain the NOT NULL, but it doesn't.

This affects other ALTER TABLE-statements too, e.g. if changing field type from IntegerField to BigIntergerField. The bug is present in 1.7 as well.