Updated handling of additional context removal

With the root context no longer being at the start of the JSON, it needs to be handled differently. Because properties are now tied to inheritance and `Thing` has the `@Context` property, we can instead use `LastIndexOf` as the way to determine where the root context information is. We use this value as the length from the start to narrow the search-and-replace space.

Author: Turnerj

Source/Schema.NET/Thing.Partial.cs

@@ -69,13 +69,13 @@ static Thing()
         /// A <see cref="string" /> that represents the JSON-LD representation of this instance.
         /// </returns>
         public string ToString(JsonSerializerOptions serializerSettings) =>
-            RemoveAllButFirstContext(JsonSerializer.Serialize(this, serializerSettings));
+            RemoveAllButRootContext(JsonSerializer.Serialize(this, serializerSettings));
 
-        private static string RemoveAllButFirstContext(string json)
+        private static string RemoveAllButRootContext(string json)
         {
             var stringBuilder = new StringBuilder(json);
-            var startIndex = ContextPropertyJson.Length + 1; // We add the one to represent the opening curly brace.
-            stringBuilder.Replace(ContextPropertyJson, string.Empty, startIndex, stringBuilder.Length - startIndex);
+            var lastIndex = json.LastIndexOf(ContextPropertyJson, StringComparison.OrdinalIgnoreCase);
+            stringBuilder.Replace(ContextPropertyJson, string.Empty, 0, lastIndex);
             return stringBuilder.ToString();
         }
     }