wps_subtitle_key filter does not affect all cases of the subtitle meta key

[benlk]

The plugin defines a filter wps_subtitle_key which changes the post meta key used to store the subtitle:

wp-subtitle/plugin/includes/subtitle.php

Lines 100 to 111 in 2e42e47

	/**
	* Get Default Subtitle
	*
	* @since 2.8
	*
	* @return string Default title.
	*/
	public function get_default_subtitle() {
	return apply_filters( 'wps_default_subtitle', '', $this->post_id );
	}

However, this filter doesn't affect the meta key used in the REST endpoint:

wp-subtitle/plugin/includes/rest.php

Line 80 in 211fb64

update_post_meta( $object->ID, 'wps_subtitle', wp_kses_post( $value ) );

There are several other places in the plugin that similarly use a hardcoded wps_subtitle meta key instead of the filter output. https://github.com/benhuson/wp-subtitle/search?l=PHP&q=wps_subtitle

[benhuson]

Searching the codebase returns many "wps_subtitle" but I think it might just be the REST reference, and the one in the is_current_subtitle() that reference the meta key and need to run through that filter.

Please check PR #55

I think this fixes the places where required?