BUG/MEDIUM: h2: don't close the connection is there are data left

h2_detach() is called after a stream was closed, and it evaluates if it's
worth closing the connection. The issue there is that the connection is
closed too early in case there's demand for closing after the last stream,
even if some data remain in the mux. Let's change the condition to check
for this.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 07a94bc..b241a5b 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -2210,11 +2210,12 @@
 		 * or sent (as seen by last_sid >= 0). A timer should be armed
 		 * to kill the connection after some idle time though.
 		 */
-		if (eb_is_empty(&h2c->streams_by_id) &&
-		    (conn_xprt_read0_pending(h2c->conn) ||
-		     (h2c->conn->flags & CO_FL_ERROR) ||
+		if (eb_is_empty(&h2c->streams_by_id) &&     /* don't close if streams exist */
+		    ((h2c->conn->flags & CO_FL_ERROR) ||    /* errors close immediately */
 		     (h2c->flags & H2_CF_GOAWAY_FAILED) ||
-		     (h2c->last_sid >= 0 && h2c->max_id >= h2c->last_sid))) {
+		     (!h2c->mbuf->o &&  /* mux buffer empty, also process clean events below */
+		      (conn_xprt_read0_pending(h2c->conn) ||
+		       (h2c->last_sid >= 0 && h2c->max_id >= h2c->last_sid))))) {
 			/* no more stream will come, kill it now */
 			h2_release(h2c->conn);
 		}