[Fix] Correct CSS duplicate property handling to use last declaration

[Fix] Correct CSS duplicate property handling to use last declaration

Fix two bugs in CSS property handling that caused text to be incorrectly
marked as invisible:

1. Fixed isset() macro misuse in override_values() - was passing a bitmask
   instead of a bit index, causing the override to never find matching values

2. Changed add_rule() to call override_values() instead of merge_values()
   when duplicate properties with normal priority are encountered, ensuring
   later CSS declarations properly override earlier ones per CSS spec

This fixes an issue where HTML emails with duplicate color declarations
(e.g., "color:#FFFFFF;color:#232333") would have text incorrectly filtered
as invisible, since only the first color was being used.

Added test case for duplicate color property handling.