ImageService v2: Fixing Create Properties (#264)

* Fix Creating Image Properties

The combination of JSON tags were not rendering the JSON body
correctly and causing a 400 error by the API.

* ImageService v2 Acceptance Tests

* unit tests
diff --git a/openstack/imageservice/v2/images/requests.go b/openstack/imageservice/v2/images/requests.go
index 32f09ee..044b5cb 100644
--- a/openstack/imageservice/v2/images/requests.go
+++ b/openstack/imageservice/v2/images/requests.go
@@ -99,7 +99,7 @@
 
 	// properties is a set of properties, if any, that
 	// are associated with the image.
-	Properties map[string]string `json:"-,omitempty"`
+	Properties map[string]string `json:"-"`
 }
 
 // ToImageCreateMap assembles a request body based on the contents of