Add add_ref parameter to intrusive_ptr::reset, add tests for reset.

include/boost/smart_ptr/intrusive_ptr.hpp

@@ -146,6 +146,11 @@ public:
         this_type( rhs ).swap( *this );
     }
 
+    void reset( T * rhs, bool add_ref )
+    {
+        this_type( rhs, add_ref ).swap( *this );
+    }
+
     T * get() const BOOST_NOEXCEPT
     {
         return px;