TSoftObjectPtr does not replicate properly

Right now USoftObjectProperty does not implement NetSerializeItem so it falls back to the implementation in UObjectPropertyBase. This only works correctly if the object happens to be in memory, otherwise it will replicate it as null. It needs to replicate the path as a string instead, which will handle both loaded and unloaded objects.

This has come up a few times from licensees such as https://udn.unrealengine.com/questions/449760/tsoftobjectptr-will-fail-to-replicate-if-object-is.html

1. Create a Replicated TSoftObjectPtr/TSoftClassPtr property in a replicated actor (either native or BP as Soft Object Reference)
2. At runtime set the ptr to an unloaded object path on the server (it doesn't have to be a real object, so /game/test.test will be fine), this will cause it to replicate to the client
3. On the client, call ToString on the softclassptr, it will return "" instead of what you set in #2.