#180 soapcpp2: union_member() can return immediately if no union types

[knut st. osmundsen]

Visual Studio pointed out that union_member() was burning a lot of cycles. Turns out we don't use any unions in VirtualBox, so we would benefit greatly from an early exit here. I measured 8% (3.81s -> 3.50s) here.

[Robert van Engelen]

That can help.