Don't use AppDomain codepath for CurrentDomain

Template hosts should return null from ProvideTemplatingAppDomain if they
don't want to use an AppDomain, but check for AppDomain.CurrentDomain
and shortcircuit that as well.

Improves usability issue discovered in #170

Mono.TextTemplating/Mono.TextTemplating/CompiledTemplate.cs

@@ -82,7 +82,7 @@ internal void SetTemplateContentForAppDomain (string content)
 		TemplateProcessor CreateTemplateProcessor ()
 		{
 			var domain = host.ProvideTemplatingAppDomain (templateContentForAppDomain);
-			if (domain == null) {
+			if (domain == null || domain == AppDomain.CurrentDomain) {
 				return new TemplateProcessor ();
 			}