Always default to boolean type for boolean fields

Fixes https://github.com/heroku/databasedotcom/issues/132
Fixes https://github.com/heroku/databasedotcom/issues/119 

`defaultValue` for boolean fields in the Salesforce API is sometimes `nil`, but saving a boolean field with a non-boolean value gives us an error:

```
value not of required type
```

Let's make sure that boolean fields are always defaulted to a boolean value.