Support non-unique meta key in autosave

[grappler]

Problem

#60

Solution

Pete mention in #24 that a filter to modify the metadata from the $_POST variable.

I looked at the PR #25 but soon realised this would not be a solution for this problem as add_metadata() needs to called multiple times for each non-unique meta data.

In addition there is a check if the data has been changed. get_post_meta() is setup to get only a single value, the first value which does not work for non-unique meta values.

wp-post-meta-revisions/wp-post-meta-revisions.php

Line 88 in ca45992

get_post_meta( $new_autosave['ID'], $meta_key, true ) !== wp_unslash( $posted_data[ $meta_key ] )

In WP 4.9.8 register_post_meta() was introduced. This allows developers to register meta key as non-unique and allows us to use the data to save the data correctly. This is how I have done it in the test:

register_post_meta( '', 'meta_revision_test', [ 'single' => false ] );

I have added tests for autosaving both unique and non-unique meta data.

[grappler]

Suggested change

	To use this plugin, you must be running WordPress 4.9.8 or newer, a function were added in 4.9.8 that are required for this implementation.
	To use this plugin, you must be running WordPress 4.9.8 or newer, a function was added in 4.9.8 that is required for this implementation.