Add BeginDetaching call for Cinder v2 (#90)

Per Cinder API, os-begin_detaching action needs to
be called prior to os-terminate_connection.
diff --git a/openstack/blockstorage/extensions/volumeactions/testing/fixtures.go b/openstack/blockstorage/extensions/volumeactions/testing/fixtures.go
index 34df070..4c3c0dd 100644
--- a/openstack/blockstorage/extensions/volumeactions/testing/fixtures.go
+++ b/openstack/blockstorage/extensions/volumeactions/testing/fixtures.go
@@ -34,6 +34,26 @@
 		})
 }
 
+func MockBeginDetachingResponse(t *testing.T) {
+	th.Mux.HandleFunc("/volumes/cd281d77-8217-4830-be95-9528227c105c/action",
+		func(w http.ResponseWriter, r *http.Request) {
+			th.TestMethod(t, r, "POST")
+			th.TestHeader(t, r, "X-Auth-Token", fake.TokenID)
+			th.TestHeader(t, r, "Content-Type", "application/json")
+			th.TestHeader(t, r, "Accept", "application/json")
+			th.TestJSONRequest(t, r, `
+{
+    "os-begin_detaching": {}
+}
+          `)
+
+			w.Header().Add("Content-Type", "application/json")
+			w.WriteHeader(http.StatusAccepted)
+
+			fmt.Fprintf(w, `{}`)
+		})
+}
+
 func MockDetachResponse(t *testing.T) {
 	th.Mux.HandleFunc("/volumes/cd281d77-8217-4830-be95-9528227c105c/action",
 		func(w http.ResponseWriter, r *http.Request) {