Make json serializer set singular alias on belongsTo

The aliases generated for the belongsTo associations are being set as plural and plural + 's' so if you have a model called 'post' it and do a belongsTo association to that model, it will create the aliases for 'posts' and 'postss' - which doesn't make much sense.

This change makes the alias be set to the singular on the association (changing ```defaultSideloadRootForType```, so when all the aliases are pluralized later it will define the pluralized alias.

I'm having issues with this as it is not_defining aliases for the singular version of some models I need, and defining the singular alias manually results on an error when it tries to pluralize the singular, as there is a key for that already.