[Authentication] Item attributes types inconsistency

[Valentin Rusu]

Hello,

Please be advised that I think that the spec has a little inconsistency
when specifying item attributes.

First, when creating an item, the signature of the CreateItem has a
"properties" arg of type "a{sv}".

Secondly, the created item has a "attributes" of type "a{ss}". 

This isn't consistent and I propose to stick with "a{sv}" as it's more
flexible, as per attached patch.

Any thoughts?

Cheers,
Valentin