That is what I initially had, because I assumed that would work too. But when an origin sends us:

    Content-Encoding: gzip
    Content-Encoding: deflate

headers_out.content_encoding will have "deflate" as it's value. 

Yet with

    Content-Encoding: foo
    Content-Encoding: gzip,deflate

headers_out.content_encoding will have "gzip,deflate" as it's value