Issue with `test_database()` helper
The code indicates that `create_tables=True`:

``` python
class test_database(object):
    def __init__(self, db, models, create_tables=True, drop_tables=True,
                 fail_silently=False):
        self.db = db
        self.models = models
        self.create_tables = create_tables
        self.drop_tables = drop_tables
        self.fail_silently = fail_silently
```

This issue seems invalid.

VIA: http://stackoverflow.com/a/33346767/254346

When using `test_database` I encountered problems with `test_db` not being initialized:

```
nose.proxy.Exception: Error, database not properly initialized before opening connection
-------------------- >> begin captured logging << --------------------
peewee: DEBUG: ('SELECT "t1"."id", "t1"."name", "t1"."count" FROM "counter" AS t1', [])
--------------------- >> end captured logging << ---------------------
```

I eventually fixed this by passing `create_tables=True` like so:

``` python
def test_timeline(self):
    with test_database(test_db, (User, Tweet), create_tables=True):
        # This data will be created in `test_db`
        self.create_test_data()
```

According to [the docs](http://docs.peewee-orm.com/en/latest/peewee/playhouse.html#test-utils) `create_tables` should default to `True` but it seems that isn't the case in the latest release of `peewee`.
