/// <summary>
 /// Update add-on
 /// </summary>
 /// <param name="id">add-on id</param>
 /// <param name="addOn">add-on to update</param>
 public Task<PlanAddOn> UpdateAddOnAsync(string id, PlanAddOn addOn)
 {
     return this.SendAsync<PlanAddOn, PlanAddOn>(
         this.CreateRequestUri(string.Format(CultureInfo.InvariantCulture, RelativePaths.AddOn, id)), 
         HttpMethod.Put, 
         addOn);
 }
 /// <summary>
 /// Create add-on
 /// </summary>
 /// <param name="addOn">add-on to create</param>
 public Task<PlanAddOn> CreateAddOnAsync(PlanAddOn addOn)
 {
     return this.SendAsync<PlanAddOn, PlanAddOn>(this.CreateRequestUri(RelativePaths.AddOns), HttpMethod.Post, addOn);
 }