Add tests for: - FilterHeaders with nil filter (uses default) - CompileHeaderFilter with empty/whitespace strings - WriteFilteredHeaders helper - Multi-value header handling Coverage: 77.8% → 97.2%
Add tests for: - FilterHeaders with nil filter (uses default) - CompileHeaderFilter with empty/whitespace strings - WriteFilteredHeaders helper - Multi-value header handling Coverage: 77.8% → 97.2%