Row not inserted into table when using CompositeKey in Meta Model
Hello,

I'm new to Peewee.

I did this code:

```
from peewee import *

db = SqliteDatabase('json_rpc.sqlite')

class RPC(Model):
    terminal_id = CharField()
    request_id = CharField()
    request = CharField()
    response = CharField()

    class Meta:
        database = db # this model uses the people database
        primary_key = CompositeKey('terminal_id', 'request_id')

# connect to our database
db.connect()

RPC.create_table()

terminal_id = 'test01'
request_id = 2
request = '{"jsonrpc": "2.0", "method": "subtract", "params": [42, 23], "id": '+ str(request_id) + '}'
response = ""
rpc = RPC(
    terminal_id=terminal_id,
    request_id = request_id,
    request=request,
    response=response
)
rpc.save()
```

Unfortunatelly row is not inserted into SQLite database (without any error).

If I comment `primary_key = CompositeKey('terminal_id', 'request_id')`
row is inserted (but there is no security to ensure that there is a unique couple (terminal_id, request_id)).

So I don't know if this is really an issue or a misunderstanding from my side.

Kind regards
