Update meta inline docs
Allow blank key or value
Don't require meta ID for prepare_meta

The ID should be in the meta_id field anyway.
Use prepare_meta in get_all_meta
Abstract edit permission check
Don't double-check in handle_post_meta_action

These cases are handled by the underlying add_meta/update_meta, so
there's no need for them here.
Trim some excess whitespace
Allow updating just the meta key too
Don't leak serialized data on get_meta
Check protected keys for update/add
Add post meta tests