"When an arbitration provision is found in an ERISA plan document itself, courts have applied state-law principles when evaluating whether the plan document contains a binding agreement to arbitrate between and among the plaintiff, the plan itself and the defendants named in the lawsuit. Yet these courts have either ignored, or not adequately addressed, the fact that ERISA itself contains rules for contract formation. The ERISA rules for contract formation -- and not state-law principles -- might govern whether an agreement to arbitrate exists in an ERISA plan document."