volume/drivers: use plugin Content-Type headers v1.2

The MediaType was changed twice in;

- b3b7eb2723461b1eb4be692f4bced0ae8ea9cb58 ("application/vnd.docker.plugins.v1+json"   -> "application/vnd.docker.plugins.v1.1+json")
- 54587d861d6664d6d32bc62a46c0c7ea0c7853e6 ("application/vnd.docker.plugins.v1.1+json" -> "application/vnd.docker.plugins.v1.2+json")

But the (integration) tests were still using the old version, so let's
use the VersionMimeType const that's defined, and use the updated version.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

volume/drivers/proxy_test.go

@@ -18,42 +18,42 @@ func TestVolumeRequestError(t *testing.T) {
 	defer server.Close()
 
 	mux.HandleFunc("/VolumeDriver.Create", func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Type", "application/vnd.docker.plugins.v1+json")
+		w.Header().Set("Content-Type", plugins.VersionMimetype)
 		fmt.Fprintln(w, `{"Err": "Cannot create volume"}`)
 	})
 
 	mux.HandleFunc("/VolumeDriver.Remove", func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Type", "application/vnd.docker.plugins.v1+json")
+		w.Header().Set("Content-Type", plugins.VersionMimetype)
 		fmt.Fprintln(w, `{"Err": "Cannot remove volume"}`)
 	})
 
 	mux.HandleFunc("/VolumeDriver.Mount", func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Type", "application/vnd.docker.plugins.v1+json")
+		w.Header().Set("Content-Type", plugins.VersionMimetype)
 		fmt.Fprintln(w, `{"Err": "Cannot mount volume"}`)
 	})
 
 	mux.HandleFunc("/VolumeDriver.Unmount", func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Type", "application/vnd.docker.plugins.v1+json")
+		w.Header().Set("Content-Type", plugins.VersionMimetype)
 		fmt.Fprintln(w, `{"Err": "Cannot unmount volume"}`)
 	})
 
 	mux.HandleFunc("/VolumeDriver.Path", func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Type", "application/vnd.docker.plugins.v1+json")
+		w.Header().Set("Content-Type", plugins.VersionMimetype)
 		fmt.Fprintln(w, `{"Err": "Unknown volume"}`)
 	})
 
 	mux.HandleFunc("/VolumeDriver.List", func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Type", "application/vnd.docker.plugins.v1+json")
+		w.Header().Set("Content-Type", plugins.VersionMimetype)
 		fmt.Fprintln(w, `{"Err": "Cannot list volumes"}`)
 	})
 
 	mux.HandleFunc("/VolumeDriver.Get", func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Type", "application/vnd.docker.plugins.v1+json")
+		w.Header().Set("Content-Type", plugins.VersionMimetype)
 		fmt.Fprintln(w, `{"Err": "Cannot get volume"}`)
 	})
 
 	mux.HandleFunc("/VolumeDriver.Capabilities", func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Type", "application/vnd.docker.plugins.v1+json")
+		w.Header().Set("Content-Type", plugins.VersionMimetype)
 		http.Error(w, "error", 500)
 	})