AnyReference::convert: fix map to struct conversion

Forging a key from struct field names was broken (we can't forge a PyObject).
The best way to fix this seems to be iterating over the map instead of the
struct.

Change-Id: If3cd8826a68866922bf724a22078eb8e49953940
Reviewed-on: http://gerrit.aldebaran.lan/51141
Reviewed-by: pdaouadi <pdaouadi@aldebaran-robotics.com>
Tested-by: pdaouadi <pdaouadi@aldebaran-robotics.com>