'AlterField should optimize into AddField.'

def test_add_field_alter_field(self):

    self.assertOptimizesTo([migrations.AddField('Foo', 'age', models.IntegerField()), migrations.AlterField('Foo', 'age', models.FloatField(default=2.4))], [migrations.AddField('Foo', name='age', field=models.FloatField(default=2.4))])
