Vary header not set when response is negotiated

[dkubb]

When the response is negotiated based on the Accept headers the Vary header is not set.

There is a comment in g07 about setting Vary but I don't think you'd set Vary for that specific use case. As a general rule you want to append any request header that is examined and results in a logic branch that affects the response.

[reiddraper]

Thanks @dkubb! We're picking back work on Airship internally, so should get to these great issues you are filing soon. Thanks.

[tmcgilchrist]

https://github.com/helium/airship/blob/master/src/Airship/Internal/Decision.hs#L384-L390