Calling Model.save() changes PaswordField passwords
That was _fast_! How do you do it? :smiley: 
 Also, thanks!

I have a `User` model with a `PasswordField` and noticed that when I change some fields can call `user.save()`, the password gets regenerated. From skimming the code, it looks like it's re-hashing the password hash.

The following is my current workaround:

``` py
from playhouse.fields import PasswordField as PasswordFieldBase, PasswordHash


class PasswordField(PasswordFieldBase):
    def db_value(self, value):
        if isinstance(value, PasswordHash):
            return value
        return super(PasswordField, self).db_value(value)
```

If it matters, I'm using `flask_utils` and `Proxy` on `master` (since it contains [the proxy fix](https://github.com/coleifer/peewee/issues/933)).
