"The purchaser was the Government and it paid the vendor for and took title to the tractors. The state law requird the vendor to collect from the vendee [here, the Government], and remit to the State, a sales tax on local sales. The vendor, at the request of the Government, paid the tax on these sales under protest and sued for refund. The State Supreme Court sustained the tax. On certiorari this Court reversed, holding that the sale was directly to the Government and that the tax was imposed directly upon the Government which was immune from state taxation."