patch 8.1.1466: not updating priority on existing sign

Problem:    Not updating priority on existing sign.
Solution:   Set the sign priority.  Add a test. (Yegappan Lakshmanan)
diff --git a/src/sign.c b/src/sign.c
index b89bea0..8f1de4a 100644
--- a/src/sign.c
+++ b/src/sign.c
@@ -330,6 +330,7 @@
 	{
 	    // Update an existing sign
 	    sign->typenr = typenr;
+	    sign->priority = prio;
 	    return;
 	}
 	else if (lnum < sign->lnum)