patch 8.1.2142: some key mappings do not work with modifyOtherKeys

Problem:    Some key mappings do not work with modifyOtherKeys.
Solution:   Remove the Shift modifier if it is already included in the key.
diff --git a/src/term.c b/src/term.c
index 27def15..e30506b 100644
--- a/src/term.c
+++ b/src/term.c
@@ -4847,8 +4847,17 @@
 		    else
 			key = arg[2];
 
-		    // insert modifiers with KS_MODIFIER
 		    modifiers = decode_modifiers(arg[1]);
+
+		    // Some keys already have Shift included, pass them as
+		    // normal keys.
+		    if (modifiers == MOD_MASK_SHIFT
+			    && ((key >= '@' && key <= 'Z')
+				|| key == '^' || key == '_'
+				|| (key >= '{' && key <= '~')))
+			modifiers = 0;
+
+		    // insert modifiers with KS_MODIFIER
 		    new_slen = modifiers2keycode(modifiers, &key, string);
 		    slen = csi_len;