Add nonce to list of restricted field keys

Hey Elliot,

I've been working to figure out an issue which was causing nonces to fail validation on the post object field.

Turns out, it's because acf_parse_types was turning the string version of a nonce into a integer, which because of strict type checking, then failed wordpress's wp_verify_nonce function.

Adding nonce to the list of restricted field keys in acf_parse_types so it isn't touched by your parse function fixes this.