Fixes DynamicNode issue not implementing IEnumerable<DynamicNode>

Adds a few unit tests for DynamicDocument/DynamicNode and fixes both to properly support the 'Take' method
as this wasn't really working. Not sure how or why it was working previously. Have also allowed for extension
methods to be executed against either of these objects that accept IQueryable which was not supported before.

src/Umbraco.Core/ApplicationContext.cs

@@ -1,8 +1,12 @@
 using System;
+using System.Collections;
 using System.Configuration;
+using System.Data;
 using System.Diagnostics;
+using System.Linq;
 using Umbraco.Core.Configuration;
 using Umbraco.Core.Logging;
+using Umbraco.Core.Models;
 
 
 namespace Umbraco.Core