Address issue with handle::cast<bool>(), quench warning in example 12 (fixes #110)

example/example12.cpp

@@ -54,6 +54,7 @@ public:
             Example12,
             run_bool
         );
+        throw std::runtime_error("this will never be reached");
     }
 
     virtual void pure_virtual() {

include/pybind11/cast.h

@@ -574,7 +574,7 @@ template <typename T> inline object cast(const T &value, return_value_policy pol
     return object(detail::type_caster<typename detail::intrinsic_type<T>::type>::cast(value, policy, parent), false);
 }
 
-template <typename T> inline T handle::cast() const { return pybind11::cast<T>(m_ptr); }
+template <typename T> inline T handle::cast() const { return pybind11::cast<T>(*this); }
 template <> inline void handle::cast() const { return; }
 
 template <typename... Args> inline object handle::call(Args&&... args_) const {