Regression in 3.12.0 with regard to saving model with JSONField
Thanks for reporting, this is now fixed in master.
A change between 3.11.2 and 3.12.0 broke support for JSONFields. This seems related to #2034, but I don't think this is a duplicate.

I'm using mysql as the database.

To reproduce:

```python
import os
import json
from peewee import Model, TextField
from playhouse.db_url import connect

uri = os.environ.get('DATABASE')
db = connect(uri)

class JSONField(TextField):

    def db_value(self, value):
        json_string = json.dumps(value)
        return json_string

    def python_value(self, value):
        if value is not None:
            return json.loads(value)


class BaseModel(Model):
    class Meta:
        database = db

class Example(BaseModel):
    data = JSONField()

e = Example.create(data=['abc', 'def'])
e.save() # FAILS for version 3.12.0; works for 3.11.2.
```

I get the following exception:
```
...
~/.local/share/virtualenvs/acm-api-UVOmdwT4/lib/python3.7/site-packages/pymysql/err.py in raise_mysql_exception(data)
    107         errval = data[3:].decode('utf-8', 'replace')
    108     errorclass = error_map.get(errno, InternalError)
--> 109     raise errorclass(errno, errval)

InternalError: (1241, 'Operand should contain 1 column(s)')
```
And, if I debug, I see the problem is with the JSONField's SQL representation:

```mysql
UPDATE `example` SET `data` = ('\"abc\"', '\"def\"') WHERE (`example`.`id` = 2)
```