Merged streams make it unable to access streams

[WoLfulus]

I'm not sure if this is intended or not, but enabling captureCombined will make getBuffers return CapturingBufferWriter for both stdout and stderr instead of PipedBuffer. The problem is that when calling the constructor of CommandChild, it checks against instances of PipedBuffer only, making it pass undefined to both buffers and making any call to CommandChild.{stdout,stderr}() fail in the assertion function.

I was expecting to be able to access the streams even when I wanted a combined buffer too.

[dsherret]

Sorry, can you say on a higher level what the issue is when using the API (not about what's going on internally because it's changing a lot) and maybe provide an example of some code erroring?