Always pass buffered channel to Read/Write
Always pass buffered channel to Read/Write

Fixes #55

c.sendRequest always does the right thing, but a few fast paths were not
constructing a buffered channel which would cause c.dispatchRequest to
block on error.

fuchsia-mirror/third_party-github.com-pkg-sftp,fuchsia-mirror/third_party-pkg-sftp,pkg/sftp,akupas/sftp,ScriptRock/sftp