fix ambiguity warning with SharedArray

As discussed here: https://github.com/JuliaLang/julia/pull/8432#issuecomment-63202199

Apologies for commit https://github.com/JuliaLang/julia/commit/1c12bcbc092b900ed71b298864cf084bb0e94971

I was trying to be fast and just edited this on Github, under the impression that this would create a pull request, rather than immediately a new commit.