Change usages of FnOnce to FnMut for serialize::Decoder.
Change usages of FnOnce to FnMut for serialize::Decoder.

brandonson/docopt.rs,birkenfeld/docopt.rs,birkenfeld/docopt.rs,zsiciarz/docopt.rs,zsiciarz/docopt.rs,birkenfeld/docopt.rs,zsiciarz/docopt.rs,brandonson/docopt.rs,brandonson/docopt.rs