Use Header.Values in GetCredentials.has

It's possibly a bit more expensive, but semantically safer
because it does header normalization.

And we'll regain the cost by not looking up the value repeatedly.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

pkg/auth/auth.go

@@ -34,8 +34,8 @@ const XRegistryConfigHeader HeaderAuthName = "X-Registry-Config"
 // the necessary authentication information for libpod operations
 func GetCredentials(r *http.Request) (*types.DockerAuthConfig, string, error) {
 	has := func(key HeaderAuthName) bool {
-		hdr, found := r.Header[key.String()]
-		return found && len(hdr) > 0
+		hdr := r.Header.Values(key.String())
+		return len(hdr) > 0
 	}
 	switch {
 	case has(XRegistryConfigHeader):