breaking change introduced by scalar div

In attempting to clean up the code for division by a scalar some unexpected breakage occurred (#493). Maybe best to revert these lines: https://github.com/JuliaMath/Polynomials.jl/pull/487/files#diff-c7d9a62e5d5a29aca07b2c1f1365565e127c595ec68316f1c1ac0da331b5c6b5L1047