fix(flag): honor full separator length when splitting map key=value pairs

splitKeyVal sliced the value from index+1, skipping only one byte of the
key/value separator. With a multi-character sep tag (e.g. sep:"=>") the
remaining separator bytes leaked into the value. Slice from index+len(sep)
so the whole separator is consumed.

flag.go

@@ -436,7 +436,7 @@ func splitKeyVal(s, sep string) (key, val string, err error) {
 	if index == -1 {
 		return s, "", nil
 	}
-	return s[:index], s[index+1:], nil
+	return s[:index], s[index+len(sep):], nil
 }
 
 func minmaxIntCheck(kind reflect.Kind, v int64) bool {