summing an empty iterator

[getzdan]

summing an empty iterator using sum() returns error instead of zero(eltype()).

discussion on google-groups: https://groups.google.com/forum/#!topic/julia-users/xLde2t7PsgU

suggested fix by Mauro:

Modifying mapflodl in reduce.jl would make sum work:

function mapfoldl(f, op, itr) 
    i = start(itr) 
    if done(itr, i) 
       return Base.mr_empty(f, op, eltype(itr)) 
        # instead of: error("Argument is empty.") 
    end 
    (x, i) = next(itr, i) 
    v0 = evaluate(f, x) 
    mapfoldl_impl(f, op, v0, itr, i) 
end