Problem/Motivation
There's no test coverage for this cache context
Proposed resolution
Add a test like \Drupal\Tests\Core\Cache\Context\QueryArgsCacheContextTest but for HeadersCacheContext. That test was added in #2729439: QueryArgsCacheContext should return a special value for ?arg (without value).
Remaining tasks
User interface changes
None.
API changes
None.
Data model changes
None.