DecimalField does not work correctly
When choosing a large type accuracy, the accuracy is not saved and the data is stored incorrectly.


```python
from peewee import *
from decimal import Decimal

class Test(Model):
    value = DecimalField(max_digits=26, decimal_places=18)

db = SqliteDatabase(':memory:')
db.bind([Test])

Test.create_table()
Test.create(value=Decimal('12345678.123456789123456789'))
row = Test.get()
print(row.value)
```

Save the number 12345678.123456789123456789
When extracting data, we get 12345678.12345679