Add handler_ptr::has_value

This function returns `true` if the container holds a managed object.

Resolves: #1079

Signed-off-by: Damian Jarek <damian.jarek93@gmail.com>

test/beast/core/handler_ptr.cpp

@@ -102,7 +102,9 @@ public:
     {
         handler_ptr<T, handler> p{handler{}};
         bool b = false;
+        BEAST_EXPECT(p.has_value());
         p.invoke(std::ref(b));
+        BEAST_EXPECT(! p.has_value());
         BEAST_EXPECT(b);
     }