Add normalizePayload to embedded-records' extractArray and extractSingle

Currently in the embedded records mixin, super is called after extracting. So, if the `normalizePayload` method is defined in the serializer's prototype it's called then. However, when normalization is needed for an entire payload prior to extracting embedded records; the extractSingle method would need to call `normalizePayload` first. The side-effect is that normalizePayload will be called twice with this addition, and is likely not so bad.