Fix for DB rows being deleted when updating post meta, closes #1107

Calling delete_metadata() with the last parameter set to true causes all meta corresponding to the specified meta_key (3nd parameter) to be deleted, ignoring the object_id (2nd parameter) ... this was causing Pods to delete all the WP native meta data for all other objects that had the same meta key.
Merge pull request #1114 from normadize/patch-1

Fix for DB rows being deleted when updating post meta, closes #1107