@Override
                public ResultCommandPair<T> call(T result) {
                    return new ResultCommandPair<T>(result, command);
                }
This class implements chaining mechanism for Hystrix commands. If a command in a chain fails, the next one
is run. If all commands in the chain failed, the error from the last one is reported.
To be able to identify the Hystrix command for which request was executed, an {@link ResultCommandPair} event
stream is returned by {@link #toResultCommandPairObservable()} method.
If information about Hystrix command is not important, {@link #toObservable()} method shall be used, which
is more efficient.

@author Tomasz Bak
ribbon/src/main/java/com/netflix/ribbon/hystrix/HystrixObservableCommandChain.java