Probably not worth making x-gzip constant.  We reference it in only one location.

But, we probably don't actually have to support it.  The only clients that really matter are the JRE's internal client, and libcurl.  I think both use "gzip" and not "x-gzip".

But, there isn't any real harm in supporting it either.  We know how to unfold what is commonly called "x-gzip", as that's just an alias for "gzip".  Code's written.  We might as well just honor it.