Parties to actions on contracts; action by beneficiary

(a) As a general rule, an action on a contract, whether the contract is expressed, implied, by parol, under seal, or of record, shall be brought in the name of the party in whom the legal interest in the contract is vested, and against the party who made it in person or by agent. (b) The beneficiary of a contract made between other parties for his benefit may maintain an action against the promisor on the contract.