Update register_meta() for WP 4.6

[pippinsplugins]

WP 4.6 is changing how register_meta() works. We need to update our use of it to match while also maintaining backwards compatibility for pre 4.6

[pippinsplugins]

See https://core.trac.wordpress.org/ticket/35658

[cklosowski]

@pippinsplugins @chriscct7 can you guys look over that commit. I've not applied the patch and things seems to still be working. Curious if I interpreted the needed updates differently than I did.

[pippinsplugins]

I'm pretty sure that is correct.

[sunnyratilal]

All looks good to me

[cklosowski]

There is an open issue #4523 where we need to do some extra sanitization of prices, I'm going include that in this issue as well.

[pippinsplugins]

@cklosowski https://make.wordpress.org/core/2016/07/20/additional-register_meta-changes-in-4-6/

[pippinsplugins]

@sunnyratilal we'll need this retested shortly once @cklosowski pushes up some changes for the adjustments just made to core.

[sunnyratilal]

Can confirm all is working:

1. Meta is being registered in the WordPress global $wp_meta_keys
2. register_meta() returns true as expected.

Good to merge methinks.

[sunnyratilal]

I'm setting up instances of WordPress 4.5.x to ensure backwards compatibility.

[sunnyratilal]

Backwards compatibility is maintained with all versions of WordPress 4.5.x (4.5 -> 4.5.3). No bugs found for me.

[cklosowski]

@sunnyratilal did you get a chance to verify that this branch also resolves #4523 ?

[pippinsplugins]

All good here.