When a bag is {{{GET}}} or {{{PUT}}} as JSON the following form is used:
{{{ { "desc": "decription of the bag (may be be an empty string or absent)", "policy": { "read": [], "write": [], "create": [], "delete": [], "manage": [], "owner": "" } } }}}
The policy may be an empty dict but the key must be present. The desc is optional.
Note that when doing a PUT what you send will fully replace anything that already exists.