fix setting None, NotLoaded and NotAvailable values on fields

weboob/capabilities/base.py

@@ -313,6 +313,7 @@ class CapBaseObject(object):
         except KeyError:
             object.__setattr__(self, name, value)
         else:
+            if value not in (NotLoaded, NotAvailable, None):
                 try:
                     # Try to convert value to the wanted one.
                     value = attr.convert(value)
@@ -321,6 +322,7 @@ class CapBaseObject(object):
                     # match the wanted following types, so we'll
                     # raise ValueError.
                     pass
+
             if not isinstance(value, attr.types) and \
                value is not NotLoaded and \
                value is not NotAvailable and \