Only accept the image manifest format (#20)

... we don't support image manifest lists yet and the public, official
docker registry recently began serving requests by clients that say
they accept it. We said we accept it but we don't actually know what
to do with it yet.

src/Network/Wreq/Docker/Registry.hs

@@ -120,9 +120,7 @@ fetchManifest = ask >>= \HockerMeta{..} ->
   where
     mkURL (ImageName n) (ImageTag t) r = C8.unpack (serializeURIRef' $ Hocker.Lib.joinURIPath [n, "manifests", t] r)
     accept = Wreq.header "Accept" .~
-      [ "application/vnd.docker.distribution.manifest.v2+json"
-      , "application/vnd.docker.distribution.manifest.list.v2+json"
-      ]
+      [ "application/vnd.docker.distribution.manifest.v2+json" ]
 
 -- | Retrieve the configuratino JSON of an image by its hash digest
 -- (found in the V2 manifest for an image given by a name and a tag).