Do not repost the entire body if not needed

[BhuzOr]

Currently editing the json_metadata of a post actually publish again the entire post.

Setting the newbody variable to something like "@@ -0 +0 @@", seems to work and avoid the entire repost.

This could also be handled by modifying steemd directly with the advantage that this trick would then be "indirectly used" by all steem libs