Missing patch to shared_ptr.hh from last commit.

Support counting even if the pointer type is converted to a compatible
base class.

Previously

class A{};
class B: public A{};

shared_ptr<A> b(new B);
shared_ptr<B> b(a);

b.get_count()==1 && b.get_count()==1 would wrongly hold true.

[[Imported from SVN: r7047]]