Bug in implementation of FSAL in oderkf.

Looking into eventually using `oderkf` also for the Bogacki–Shampine method (aka `ode23`), I found that the "First Same As Last " (FSAL) property of the Dormand-Prince pair is not correctly implemented. The error was already present in the "source" [ode45](http://users.powernet.co.uk/kienzle/octave/matcompat/scripts/ode_v1.11/ode45.m), but has been [corrected](https://sites.google.com/site/comperem/home/ode_solvers).

Essentially, `k[1,:] = k[end,:]` should only be done if the integration step was accepted. I also took the liberty to remove an unnecessary check of `delta==0`.

(cc: @pao)