Doctype alias validation always fails

As it only checks if there is a doctype with the same alias, but does
not test if this doctype is the same as the one currently being saved.

src/Umbraco.Web/Editors/ContentTypeControllerBase.cs

@@ -191,7 +191,7 @@ namespace Umbraco.Web.Editors
             
             //Validate that there's no other ct with the same name
             var exists = getContentTypeByAlias(contentTypeSave.Alias);
-            if (exists != null)
+            if (exists != null && exists.Id != ctId)
             {
                 ModelState.AddModelError("Alias", "A content type with this alias already exists");
             }