Fix for embedded non-collection resources

Hey @sandersonet, sorry for not stumbling on this earlier.

Problem 1 is that HyperResource accepts hashes and arrays for `_embedded`, but chokes on `null` entries. We make that more graceful.

Problem 2 is that the `embeds_one` relationship has to be a bit different than `has_one` / `belongs_to`, since it's accessing the `_embedded` subdocument and not the `_links` subdocument.