Fix ASP0015: Suggest using IHeaderDictionary properties

Author: bkoelman

src/JsonApiDotNetCore/Controllers/BaseJsonApiController.cs

@@ -214,7 +214,7 @@ public virtual async Task<IActionResult> PostAsync([FromBody] TResource resource
 
         if (newResource == null)
         {
-            HttpContext.Response.Headers["Location"] = locationUrl;
+            HttpContext.Response.Headers.Location = locationUrl;
             return NoContent();
         }