Fixed issue preventing properties having unreserved underscored names.

Unreserved underscored properties are now accessible when reading json resources.

This is inline with the spec, see:
http://tools.ietf.org/html/draft-kelly-json-hal-05#appendix-B.4

> B.4.  Are all underscore-prefixed properties reserved?
> No, HAL only reserves the names detailed in this specification."