This commit addresses an issue where cached responses for PlexTV API
requests could return incorrect user data when multiple users were logged in.
This was caused by a shared cache key that did not account for differences
in auth tokens, i.e `X-Plex-Token`. The `serializeCacheKey` method should
now include headers in the cache key generation to ensure unique cache keys.
This should fix the plex user mismatch that occurred due to the same
cache key being used without accounting for the difference in auth
token.

fix #1227