v0.13.0

Added

• conditional-requests-api, a "Descriptions" group: a Collection PUT with
  If-None-Match: * creates an absent Collection and 412s on a present one;
  Read Space carries a quoted ETag and a covering If-None-Match is 304
  (optional, the spec's SHOULD); a Space PUT with If-None-Match: * creates
  an absent Space and 412s on a present one; a stale If-Match on Update
  Space 412s while the current one succeeds and an unconditional PUT still
  replaces. The write cases skip when the default backend does not advertise
  conditional-writes.