send the end cell when we realize we're going to end,

send the end cell when we realize we're going to end,
not when we're closing the stream.

this lets us put a payload in the end cell if we want to,
to describe why we're closing the stream.

there are still some places where we don't send the end cell
immediately. i need to track them down. but it's a low priority,
since i've made it send the end cell when we close the stream if
we haven't already sent it.

svn:r640