Always default to boolean type for boolean fields

defaultValue for boolean fields in the Salesforce API is 'nil', but
saving a boolean field with that value gives us an error:

```
value not of required type
```

Instead of defaulting to nil, we make sure that the default value is
always converted to a boolean for boolean fields.
Merge pull request #1 from chrishunt/boolean-defaults

Always default to boolean type for boolean fields