This is completely bizarre. Assange is not currently wanted in the US. There is no indictment against him, there isn't a warrant for his arrest, and there is no request for extradition. I'm not sure how he can "agree to go to prison" when he hasn't been charged with a crime. The U.S. doesn't actually let people go to prison just because they want to; they have to be found guilty of a crime.
Assange is wanted in Sweden (although so far he's only wanted for questioning.)
Assange is wanted in Britain-- for jumping bail.
But he's not wanted for a crime in the U.S. He could agree to go to prison in Sweden or Britain-- why doesn't he volunteer to do that?