A failing CORS test in the gate discovered that we were responding
application/json to ?format=txt requests (which is maybe not even a
valid value for that qs param?), but only when running with
eventlet==0.38.0
This avoids the problem of backend container server HEADs no longer
having 'Content-Length: 0' by fixing the client HEAD resp headers before
we check for chunked-transfer resp.
Drive-By: refactor listing_formats to use HeaderKeyDict and always set
Content-Length explicitly
Co-Authored-By: Alistair Coles <alistairncoles@gmail.com>
Co-Authored-By: Matthew Oliver <matt@oliver.net.au>
Change-Id: If724485e1425d1481d10b9255436301e346f07e8
fa889358ac