By David Morgan WASHINGTON (Reuters) -Democratic U.S. Senator Raphael Warnock and Republican challenger Herschel Walker sparred over a range of issues from abortion and policing to personal integrity on Friday, as a key Georgia contest that could help determine control of the Senate came to a head in a contentious televised debate. Walker, a one-time football star and political novice backed by former President Donald Trump, sought to brand the incumbent as a rubber stamp for Democratic President Joe Biden’s agenda, which Republicans have painted as responsible for inflation, crime and other s…