foreach would remove 1 item when validating generators

[tinman6]

When validating generators with foreach, _convert_to_list would remove 1 item.

This is the culprit:

for n in value:
    break
return value

Our workaround is to override _convert_to_list with the following:

def _convert_to_list(self, value):
    if isinstance(value, types.GeneratorType):
        return list(value)
    else:
        return super(ListOf, self)._convert_to_list(value)