Refactor Layer interface to return a Handler

... Rather than ServeHTTP directly.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

registry.go

@@ -108,10 +108,10 @@ type Layer interface {
 	// CreatedAt returns the time this layer was created.
 	CreatedAt() time.Time
 
-	// ServeHTTP allows a layer to serve itself, whether by providing
-	// a redirect directly to the content, or by serving the content
-	// itself
-	ServeHTTP(w http.ResponseWriter, r *http.Request)
+	// Handler returns an HTTP handler which serves the layer content, whether
+	// by providing a redirect directly to the content, or by serving the
+	// content itself.
+	Handler(r *http.Request) (http.Handler, error)
 }
 
 // LayerUpload provides a handle for working with in-progress uploads.