Imply on bed -> bed

BUR #611 is pending approval.

create implication on_bed -> bed

For someone to be able to be on bed, a bed must be in the image.

Related: https://danbooru.donmai.us/forum_topics/20909

If tag was used correctly as described in the wiki, and not for posts that should be only tagged bed sheet and such instead, then BUR should be approved. As it stands, danbooru decision is right.