Update hashdict.jl

Without this change, the following code example fails.  Not sure how to add a test:

using DataStructures
 
od = OrderedDict{Int,Int}()
od[1] = 2
 
ranges = [2:5,6:9,10:13]
for range in ranges
    for i = range
        od[i] = i+1
    end
 
    for i = range
        delete!( od, i )
    end
end
od[14]=15