By Mike Stone WASHINGTON (Reuters) – Congressional Democrats, including Senator Elizabeth Warren, have asked the U.S. Commerce Department to curb assault weapons exports and increase oversight of gun exports after a Trump-era ruling to ease firearms export laws pushed sales up, according to a letter sent Wednesday and seen by Reuters. The letter, which was signed by Warren, Senator Chris Murphy and U.S. Representative Joaquin Castro, was sent to Commerce Secretary Gina Raimondo. The lawmakers wrote that they have “grave concern about Commerce Department actions that have weakened oversight of …