The update of BooleanField coerce
Is this in reference to a particular issue?
 **Amazing！**
 This patch seems more suitable for a subclass of BooleanField that is designed to handle things like strings or untrusted data.

Good afternoon, coleifer. We've found the most common case is that the data from the client(as browser) will send true or false, 1 or 0 to the server, but the conversion should be decided by peewee on the data field type. So may be the update of mine makes common sense.
