Fix OnReturn secondary resource pipeline for HasOne relationships

[maurei]

Additional fix for #924 that covers the case of HasOne relationships

[woofi7]

I've pulled the branch and tested on my side with the project.

When tested it crashes on the call to OnReturn.

The var resources is of type List<object> which is not TResource

Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: The type 'object' cannot be used as type parameter 'TResource' in the generic type or method 'JsonApiDotNetCore.Hooks.Internal.IOnReturnHookExecutor.OnReturn<TResource>(System.Collections.Generic.IEnumerable<TResource>, JsonApiDotNetCore.Hooks.Internal.Execution.ResourcePipeline)'. There is no implicit reference conversion from 'object' to 'JsonApiDotNetCore.Resources.IIdentifiable'.
  at CallSite.Target
  at System.Dynamic.UpdateDelegates.UpdateAndExecute3
  at JsonApiDotNetCore.Hooks.Internal.ResourceHookExecutorFacade.OnReturnRelationship
    D:\workspace\JsonApiDotNetCore\src\JsonApiDotNetCore\Hooks\Internal\ResourceHookExecutorFacade.cs:131
  at JsonApiDotNetCore.Services.JsonApiResourceService`2+<GetSecondaryAsync>d__11.MoveNext
    D:\workspace\JsonApiDotNetCore\src\JsonApiDotNetCore\Services\JsonApiResourceService.cs:137

I have tried with this and it works:

if (resourceOrResources is IIdentifiable)
{
    var resources = ToList((dynamic) resourceOrResources);
    object[] array = _resourceHookExecutor.OnReturn(resources, ResourcePipeline.GetRelationship).ToArray();
    return array.SingleOrDefault();
}

This do not work, we need to store the result in a var and then do the SingleOrDefault.
_resourceHookExecutor.OnReturn(resources, ResourcePipeline.GetRelationship).ToArray().SingleOrDefault()

[woofi7]

The fix work with the first problem (image loading).