Back to the original {add:true} on fetch

There has been a little bit of confusion around how `add:true` should work with the addition of the `update` API (#2008, #1975, #2009). 

The `{add:true}` is a common enough case that it should also prevent the items from being removed - rather than needing to specify `{update:true, remove:false}`.

This pull request gives `collection.fetch({add:true})` the same implementation it had before, without affecting the `update` api call, with the ability to specify additional options such as `{merge:false}` if desired.