Ensure @cached_options is initialized

The current implementation is generating a warning because `@cached_options` hasn't been initialized when it's first called. 

```
~/.gem/ruby/1.9.3/gems/multi_json-1.7.7/lib/multi_json.rb:10: warning: instance variable @cached_options not initialized
```

Using the `||=` operator prevents this warning, even though it seems like its doing the assignment twice. Not sure if there's a better option?