update_post_meta should add nonexistent values

[filosofo]

Currently, if you try to use update_post_meta to update a post meta key and value that don't exist, it just returns false. This behavior is at odds with that of other update functions such as update_option, update_user_option, update_usermeta, wp_update_user, wp_update_category, and update_recently_edited, all of which will create the updated object, if it doesn't already exist.

The other updating functions, which like update_post_meta fail when the object doesn't already exist, would not typically be called when one is agnostic about the existence of the object in question: wp_update_comment and update_attached_file, e.g.; wp_update_term seems to be a possible exception.

Because update_post_meta is semantically similar to update_option and update_usermeta and like them is frequently employed in plugins, it should behave similarly.

[ryan]

(In [7978]) Add non-existent values instead of returning g false in update_post_meta(). Props filosofo. fixes #7018

[mdawaffe]

update_meta() is now double escaping the meta_value.

$wpdb->update( does escaping for you; you don't need to call $wpdb->escape() first.

[ryan]

(In [7980]) Don't double escape. fixes #7018

[filosofo]

I've added a patch that removes now-unnecessary checks for the existence of post meta data.

[ryan]

(In [7985]) Remove unnecessary checks for the existence of post meta data. Props filosofo. fixes #7018

[azaozz]

Using update_post_meta() instead of add_post_meta() converts the array of attachment properties to a string when uploading new image. That results in php warnings on the Media Library page.

The patch slightly optimizes update_post_meta() and ensures it calls add_post_meta() when the key doesn't exist.

[ryan]

(In [7995]) Fix serialization when adding meta via update_post_meta(). Props azaozz. fixes #7018