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.
Whoops. Nice catch :)