Bug with multiple foreign keys to same table and DeferredRelation
Fixed! Thanks for the great ticket.
 Glad to be of service :)

Python3 with Mysql, peewee from pip3 install --upgrade peewee. Two mysql tables:

```
mysql> desc c ;
+-------+---------+------+-----+---------+----------------+
| Field | Type    | Null | Key | Default | Extra          |
+-------+---------+------+-----+---------+----------------+
| id    | int(11) | NO   | PRI | NULL    | auto_increment |
+-------+---------+------+-----+---------+----------------+
1 row in set (0.00 sec)

mysql> desc p ;
+-------+---------+------+-----+---------+----------------+
| Field | Type    | Null | Key | Default | Extra          |
+-------+---------+------+-----+---------+----------------+
| id    | int(11) | NO   | PRI | NULL    | auto_increment |
| c1    | int(11) | YES  |     | NULL    |                |
| c2    | int(11) | YES  |     | NULL    |                |
+-------+---------+------+-----+---------+----------------+
3 rows in set (0.01 sec)
```

and the following test code:

```
from peewee import *
db = MySQLDatabase('p', user='p', password='p', host='localhost')
deferredC = DeferredRelation()

class C(Model):
    id = PrimaryKeyField()
    class Meta:
        db_table = 'c'
        database = db
class P(Model):
    id = PrimaryKeyField()
    c1 = ForeignKeyField(deferredC, db_column='c1', related_name='c1')
    c2 = ForeignKeyField(deferredC, db_column='c2', related_name='c2')
    class Meta:
        db_table = 'p'
        database = db

deferredC.set_model(C)

c1 = C.create()
c2 = C.create()
p1 = P.get(P.id == 3)
p1.c1 = c1
p1.save()
p2 = P.get(P.id == 3)
p2.c2 = c2
p2.save()
db.commit()
```

If I run this on database with table c empty, and one record in p with p.c1=null, p.c2=null, then I get two records in table c (as expected) but only p.c1 links, not p.c2:

```
mysql> select * from c ;
+----+
| id |
+----+
| 15 |
| 16 |
+----+
2 rows in set (0.00 sec)

mysql> select * from p ;
+----+------+------+
| id | c1   | c2   |
+----+------+------+
|  3 |   15 | NULL |
+----+------+------+
1 row in set (0.00 sec)
```

If I remove the DeferredRelation() and use ForeignKeyField(C, ....) then it works as expected, and p.c1 and p.c2 are both correctly set.

Thanks
Mike
