Mapistore rest fixes

This pull request includes a fix in the MAPIStore Python bindings and some corrections in the REST API.

* There were some remaining magic values in _rest.py_ that could be replaced by values in the MAPIStore Python bindings.
* In the @set_properties@ methods, the dictionary update could be done more efficiently.
* Several functions unnecessarily modified their arguments, which is error-prone.
@jkerihuel `set_restrictions` is plural and `_encode_restriction` is singular. IHMO, one contradicts the other... Futhermore, the `_encode_restriction` result seems like a `dict` but the default value is an empty list `[]`...