close the connection on partial writes

fixes #7 (not the best way but it works)

multiplex.go

@@ -143,18 +143,22 @@ func (mp *Multiplex) sendMsg(header uint64, data []byte, dl time.Time) error {
 	n += binary.PutUvarint(buf[n:], uint64(len(data)))
 	n += copy(buf[n:], data)
 
-	_, err := mp.con.Write(buf[:n])
-	if err != nil {
+	written, err := mp.con.Write(buf[:n])
+	if err != nil && written > 0 {
+		// Bail. We've written partial message and can't do anything
+		// about this.
+		mp.con.Close()
 		return err
 	}
 
 	if !dl.IsZero() {
-		if err := mp.con.SetWriteDeadline(time.Time{}); err != nil {
-			return err
+		// only return this error if we don't *already* have an error from the write.
+		if err2 := mp.con.SetWriteDeadline(time.Time{}); err == nil && err2 != nil {
+			return err2
 		}
 	}
 
-	return nil
+	return err
 }
 
 func (mp *Multiplex) nextChanID() uint64 {