Test all Vary Headers

[barryvdh]

Resolves #105 Replaced #107

When multiple Vary headers are set, Symfony only returns the first one. When replacing the Vary header, it removes the 2nd one. This changes behavior so that in the case multiple headers are used, it check all existing Vary headers and appends it as a new header. This will result in an additional Vary header but only when there were already 2 or more.