omit refId in incapsula_site_v3 when marshaling to JSON if the field has no value (#575)

* clarify site versioning

* remove global field and use it as string

* Allow deletion of the last domain for v1 sites that have migrated to the v3 resource.

* Allow deletion of the last domain for v1 sites that have migrated to the v3 resource.

* Allow deletion of the last domain for v1 sites that have migrated to the v3 resource.

(cherry picked from commit 976b940)

* Allow deletion of the last domain for v1 sites that have migrated to the v3 resource.

(cherry picked from commit 5e35300)

* omit refId when marshaling to JSON if the field has no value.

Author: adiEdriShlomo

incapsula/client_site_v3.go

@@ -20,7 +20,7 @@ type SiteV3Request struct {
 	Cname        string `json:"cname,omitempty"`
 	Name         string `json:"name,omitempty"`
 	SiteType     string `json:"type,omitempty"`
-	RefId        string `json:"refId"`
+	RefId        string `json:"refId,omitempty"`
 	Active       bool   `json:"active"`
 }