By Miho Uranaka and Sam Nussey TOKYO (Reuters) -Japanese e-commerce and finance heavyweight Rakuten is weighing an initial public offering in the United States of its credit card business, according to two sources familiar with the matter. Rakuten began considering a potential U.S. listing of one of Japan's largest credit card businesses last month, the sources said. The considerations are in the early stages, with other potential options including a stake sale to a strategic buyer, one of the sources sa ...