Buggy connections could crash node.js. Now check connection before sending data every time

http://groups.google.com/group/nodejs/browse_thread/thread/16abfa87c32408f3

We have our node.js server monitored by monit, however it seems monit is pretty
agressive / quick about closing its connection and thus we've gotten into a
loop of errors like this:

    at #<a ServerResponse>.flush
    at #<a ServerResponse>.sendBody
    at [object Object].json
    at [object Object].[anonymous]
    at [object Object].[anonymous]
    at [object Object].[anonymous]
http.js:353: Socket is not open for writing
      connection.send(out, out.encoding);
                 ^

Below is a patch that basically cause flushMessageQueue to check the connection
state for each item in the queue rather than just a single time in the
beginning.