Separate cache and http request into two separate Hystrix commands.

We can also move this method to the command itself and possibly create a parent command class for both RibbonHystrixObservableCommand and HystrixCacheObservableCommand. Then the method in CommandChain class is only responsible for chaining the two commands, but not transforming the Observable. What do you think?

I also suggest that we rename RibbonHystrixObservableCommand to HttpResourceObservableCommand and rename HystrixCacheObservableCommand to CacheObservableCommand.