Dev/ar 6 : Serialization bug fixes

Hi Kevin,

Sorry about the delay. This branch contains one fix - it should merge cleanly into your develop branch.

Fix - 
(1) deal with empty archetype ```{"fieldsets": []}```

Relevant diff: (approximately...)
```csharp
--- a/app/Umbraco/Umbraco.Archetype/Serialization/ArchetypeJsonConverter.cs
+++ b/app/Umbraco/Umbraco.Archetype/Serialization/ArchetypeJsonConverter.cs
@@ -136,15 +136,15 @@ namespace Archetype.Serialization
             if (objToken.SelectToken("properties") != null)
                 return PopulateProperties(obj, objToken["properties"]);
 
-            if (objToken is JArray) 
-            {
-                var defaultFsProperties = properties.Where(pInfo => !HasAsFieldsetAttribute(pInfo)).ToList();                
+            if ((objToken is JArray) && (objToken as JArray).Count <= 0)
                 return null;
 
+            if (!(objToken is JArray))
+                return PopulateProperties(obj, new JArray(objToken));
+
 
-                foreach (var property in defaultFsProperties)
-                {
-                    var propJToken = ParseJTokenFromItems(objToken, GetJsonPropertyName(property));
-                    PopulateProperty(obj, propJToken["properties"], property);
-                }                
+            var defaultFsProperties = properties.Where(pInfo => !HasAsFieldsetAttribute(pInfo)).ToList();                
+
+            foreach (var property in defaultFsProperties)
+            {
+                var propJToken = ParseJTokenFromItems(objToken, GetJsonPropertyName(property));
+                PopulateProperty(obj, propJToken["properties"], property);
             }
 
             return PopulateProperties(obj, new JArray(objToken));
```

And there are couple of new tests to check for this condition:
Umbraco/Archetype.Tests/Serialization/Regression/ArchetypeJsonConverterLegacySetUpTest
Umbraco/Archetype.Tests/Serialization/Regression/JsonTestLegacyModelSetUp.cs
Umbraco/Archetype.Tests/Serialization/Regression/JsonTestStrings.cs
Umbraco/Archetype.Tests/Serialization/Regression/ArchetypeJsonConverterTest.cs > EmptyArray_Returns_Null()

Cheers,