Warning: Explicitly defaulted move assignment operator is implicitly deleted

[dhruvkakadiya]

Hello, thank you for this awesome tool on top of beast.

While building for Mac OS(clang-13.1), noticed below warning. So seems like either explicitly defaulted functions at server.hpp can be removed or application.hpp needs to be fixed.

~/beauty/include/beauty/server.hpp:64:13: warning: explicitly defaulted move assignment operator is implicitly deleted [-Wdefaulted-function-deleted]
    server& operator=(server&&) = default;
            ^
~/beauty/include/beauty/server.hpp:100:29: note: move assignment operator of 'server' is implicitly deleted because field '_app' is of reference type 'beauty::application &'
    beauty::application&    _app;
                            ^

[dfleury2]

Done move assignement is now deleted.