patch 8.1.0803: session file has problem with single quote in file name

Problem:    Session file has problem with single quote in file name. (Jon
            Crowe)
Solution:   Use a double quoted string.  Add a test.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index f60bb34..210fdfa 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -11838,9 +11838,9 @@
 	     * edit that buffer, to not lose folding information (:edit resets
 	     * folds in other buffers)
 	     */
-	    if (fputs("if bufexists('", fd) < 0
+	    if (fputs("if bufexists(\"", fd) < 0
 		    || ses_fname(fd, wp->w_buffer, flagp, FALSE) == FAIL
-		    || fputs("') | buffer ", fd) < 0
+		    || fputs("\") | buffer ", fd) < 0
 		    || ses_fname(fd, wp->w_buffer, flagp, FALSE) == FAIL
 		    || fputs(" | else | edit ", fd) < 0
 		    || ses_fname(fd, wp->w_buffer, flagp, FALSE) == FAIL