simplify bson_encode arg validation to workaround a bug
simplify bson_encode arg validation to workaround a bug

Adding the "InstanceOf" clause seems to break on Perl before 5.14.
I'm not sure why, but loosening the validation fixes it, and
the document's type gets validated during encoding anyway.

gormanb/mongo-perl-driver,kainwinterheart/mongo-perl-driver,xdg/mongo-perl-driver,mongodb/mongo-perl-driver,rahuldhodapkar/mongo-perl-driver,kainwinterheart/mongo-perl-driver,kainwinterheart/mongo-perl-driver,gormanb/mongo-perl-driver,rahuldhodapkar/mongo-perl-driver,jorol/mongo-perl-driver,gormanb/mongo-perl-driver,mongodb/mongo-perl-driver,dagolden/mongo-perl-driver,dagolden/mongo-perl-driver,rahuldhodapkar/mongo-perl-driver,dagolden/mongo-perl-driver,dagolden/mongo-perl-driver,kainwinterheart/mongo-perl-driver,jorol/mongo-perl-driver,jorol/mongo-perl-driver,xdg/mongo-perl-driver,rahuldhodapkar/mongo-perl-driver,mongodb/mongo-perl-driver,xdg/mongo-perl-driver,gormanb/mongo-perl-driver,jorol/mongo-perl-driver