sizehint for Sets now returns the given Set

Minor patch, if I want to contribute I have to start somewhere.

sizehint previously returned the Dict as-per [discussion](https://groups.google.com/forum/#!topic/julia-users/COdDQwrgKQQ).

Before:

    julia> sizehint(Set(), 4711)
    Dict{Any,Nothing}()

After:

    julia> sizehint(Set(), 4711)
    Set{Any}({})