The function `Request.check` did not work properly due to
`Container.contains_all` performing the check with the operands swapped.
Fix the call and improve the body of `ObjectSet.issubset`.
Also improve the error message, highlighting the missing objects.
Overload the `get` function of `Requests` so that, in addition to the
usual `Mapping` behavior, will return an empty `ObjectSet` if no default
is specified.
Giacomo Vercesi