Fix 4.1 adapter under Rubinius

Calling return inside a block under Rubinius results in aLocalJumpError, at least with version 2.3.0. This is probably a Rubinius bug but it is easy enough to work around it here.

I am making the assumption that accept! will never return nil. Is that reasonable?

Unfortunately one other spec still fails but I don't think it's Squeel's fault. It's something to do with preloading HABTM associations, possibly caused by the Rubinius implementation of flat_map.