By Louis Charbonneau UNITED NATIONS - John McCain’s U.S. presidential election campaign has solicited a financial contribution from an unlikely source — Russia’s U.N. envoy — but a McCain spokesman said on Monday it was a mistake. In the letter, McCain urged Russia’s U.N. Ambassador Vitaly Churkin to contribute anywhere from $35 to $5,000 to help ensure McCain’s victory over Democratic rival Sen. Barack Obama, currently ahead in voter preference polls. “If I have the honor of continuing to serve you, I make you this promise: We will always put America — her strength, her ideals, her future — before...