(plaintiff brought a putative class action on behalf of similarly situated Minnesota counties seeking a declaratory judgment that Fannie Mae and Freddie Mac had violated state law by failing to pay transfer tax on transfers of deeds to real property; the County sought recovery for unjust enrichment and injunctive relief; the district court granted the defendants’ motion to dismiss for failure to state a claim, the County appealed, and the appellate court affirmed; the court ruled that the federal agencies’ charters stated that the agencies "shall be exempt from all taxation . . . imposed by any State," and identified their real property as the sole exception to the general rule; the agencies were governmental instrumentalities which Congress had the authority to protect by exempting them from taxation imposed by the states; because the federal entities were not obliged to pay the tax, no unjust enrichment claim was stated).