Make insecure construction explicit, to address #2614

src/ruby/ext/grpc/rb_server.c

src/ruby/spec/client_server_spec.rb

src/ruby/spec/generic/active_call_spec.rb

src/ruby/spec/generic/client_stub_spec.rb

src/ruby/spec/generic/rpc_server_spec.rb

src/ruby/spec/pb/health/checker_spec.rb

src/ruby/spec/server_spec.rb