Playboy Playmate Karissa Shannon says it's true that she made a sex tape with Heidi Montag — but Montag's estranged husband Pratt had nothing to do with it. "There is a sex tape of me and Heidi," Shannon tells Radaroline, adding that Pratt, 26, was not present for the filming. "It's just me and Heidi, that's it." Last weekend, Pratt said that he found the X-rated footage while going through things at the Malibu home he once shared with Montag, 23. According to Shannon, he not only stole the tape but the video camera in which it was found. "He...