Jeremy Strohmeyer, 19, changed his plea to guilty at the last minute yesterday to raping and strangling a 7 year old girl in a casino bathroom, to avoid the death penalty.  Under a plea bargain struck just before opening statements, were to have begun, he will spend the rest of his life behind bars for the murder of Sherrice Iverson in the Primmadonna Casino in May 1997.  Strohmeyer said he kidnapped, molested and strangled Sherrice.  The decision to seek a plea bargain was reached Friday, when prosecutors said they had evidence that 30 hours before he killed Sherrice, he said, I fantasize about having sex with 5 and 6 year old girls all the time, while in an Internet chat room. Sherrice was wandering around the casino late at night while her father gambled when Strohmeyer saw her.  Her father ignored security guards requests three times to take care of his daughter.  Strohmeyer's friend, David Case, Jr., later told cops he took no action after seeing him holding Sherrice in a rest room stall, muffling her screams.  He said he told him to let her go and then left the rest toom.  The case remains controversial because of this.  Cash insists he had no responsibility to do anything further and told the Los Angeles Times the notoriety made it easier for him to score with women.