convert(Vector{Int}, o) is much slower than copy(PyVector{Int}(o))

As discussed [on julia-users](https://groups.google.com/d/msg/julia-users/1M8wDT0e5h4/qZ2OPaM9Hi4J), the two pathways to converting a Python list to a `Vector{Int}` seem mysteriously different in performance. For example:

``` julia
iv = PyVector(rand(Int16, 10^6))
@time copy!(Array(Int, length(iv)), iv);
@time convert(Vector{Int}, iv.o);
```

yields 3-5× slower performance for the second version on my machine.