Fix MarbleProcessor's fall through switch cases

The build of RSocket fails in this Ubuntu 18.04 Docker environment. The reason is the fall through switch statements.

/home/rsocket-cpp/rsocket/tck-test/MarbleProcessor.cpp:95:33: warning: this statement may fall through [-Wimplicit-fallthrough=]
         subscriber.onError(std::runtime_error("Marble Error"));
                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/rsocket-cpp/rsocket/tck-test/MarbleProcessor.cpp:96:7: note: here
       case '|':
       ^~~~
/home/rsocket-cpp/rsocket/tck-test/MarbleProcessor.cpp:98:30: warning: this statement may fall through [-Wimplicit-fallthrough=]
         subscriber.onComplete();
         ~~~~~~~~~~~~~~~~~~~~~^~
/home/rsocket-cpp/rsocket/tck-test/MarbleProcessor.cpp:99:7: note: here
       default: {

As long as the marble_ has more items and more messages are expected from the subscriber, it will continue publishing to the subscriber.

rsocket / rsocket-cpp

Fix MarbleProcessor's fall through switch cases #891