Discrepancy between send and delayed_send for groups

[rkfg]

Given the example from #632, consider the following code:

    scoped_actor self(system);
    self->delayed_send(grp, 1s, "test");
    self->send(grp, "test");

The delayed_send line doesn't compile:

In file included from /opt/build/static/include/caf/send.hpp:25:0,
                 from /opt/build/static/include/caf/all.hpp:27,
                 from ../src/main.cpp:1:
/opt/build/static/include/caf/actor_cast.hpp: In instantiation of ‘constexpr const bool caf::{anonymous}::is_weak_ptr<caf::group>::value’:
/opt/build/static/include/caf/actor_cast.hpp:173:18:   required from ‘T caf::actor_cast(U&&) [with T = caf::intrusive_ptr<caf::actor_control_block>; U = const caf::group&]’
/opt/build/static/include/caf/mixin/sender.hpp:142:60:   required from ‘void caf::mixin::sender<Base, Subtype>::delayed_send(const Dest&, const caf::duration&, Ts&& ...) [with caf::message_priority P = (caf::message_priority)0; Dest = caf::group; Ts = {const char (&)[5]}; Base = caf::scheduled_actor; Subtype = caf::event_based_actor]’
src/subdir.mk:18: recipe for target 'src/main.o' failed
/opt/build/static/include/caf/mixin/sender.hpp:151:17:   required from ‘void caf::mixin::sender<Base, Subtype>::delayed_send(const Dest&, std::chrono::duration<Rep, Period>, Ts&& ...) [with caf::message_priority P = (caf::message_priority)0; Rep = long int; Period = std::ratio<1>; Dest = caf::group; Ts = {const char (&)[5]}; Base = caf::scheduled_actor; Subtype = caf::event_based_actor]’
../src/main.cpp:21:43:   required from here
/opt/build/static/include/caf/actor_cast.hpp:64:25: error: ‘has_weak_ptr_semantics’ is not a member of ‘caf::group’
   static constexpr bool value = T::has_weak_ptr_semantics;
                         ^~~~~
make: *** [src/main.o] Error 1

While the next line compiles fine and works as expected. I don't see much semantic difference between the two so I suppose the both cases should be possible and it's a bug.