Fix process_tags overwriting existing tags

[lemacarl]

Currently when one tries to add tags to a post by hooking into the o2_create_post the tags are overwritten by process_tags which overwrites any existing tags.

[mdawaffe]

Thanks for the suggestion!

With this change, tags cannot be removed by deleting the #inline-tags in the o2 post editor.

Rather that changing o2 (which has complicated tag handling), is it possible to make your code work by hooking into o2_writeapi_post_created instead of o2_create_post?