FaultRecoveringOutputStream.

This is the first step in a change that'll permit the underlying
socket stream to change out-from-under-you while you're uploading
POST data to a server.

It'll recover from failures writing the request body that occur
within the first N bytes, where N will be determined by the number
of bytes that can be written without positive confirmation.

See https://github.com/square/okhttp/issues/137