encodeWithCoder properly encodes has_one relationships as well

When serializing objects that have a has_one relationship, the following error occurs:  

```ruby
NSInvalidArgumentException: -[ArrayFinderQuery encodeWithCoder:]: unrecognized selector sent to instance 0xb659440 (RuntimeError)
```

By ignoring has_one in encodeWithCoder, we avoid attempting to serialize the relationship.