Fix object_id with padding

[carnaval]

Forgotten in the padding fix : immutables with haspadding fields.

julia> immutable K; x::Bool; y::Int; end
julia> immutable K3; x::K; end
julia> object_id(K3(K(false,1)))
0x2816178d071f22f3
julia> object_id(K3(K(false,1)))
0x7eaed272352f26d8

turns out Dict don't like that.

[tkelman]

can you add a test?

[carnaval]

done

[JeffBezanson]

Thank you!

The test can be made more reliable using the method of the test for #11813.