Ambiguity between "operation cancelled" and "0-length operation completed"

[alexcrichton]

Assuming #444 with respect to 0-length reads writes there's an ambiguity when cancelling an operation between whether the operation was cancelled or wheter the 0-length operation completed. This was discussed briefly at #444 (comment) and it sounds like there are encoding changes planned to address this. Regardless though I wanted to open an issue here to write a FIXME in some code, so this is basically a tracking issue for said encoding changes.