Store methods behaves inconsistently when persistence constraints specified

The Bucket#set for example should return the object corresponding to arguments passed:

1.9.3p327 (main):002:0> conn.set("foo", "bar")
851339802448297984
1.9.3p327 (main):003:0> conn.set("foo" => "bar", "baz" => "foo")

{"foo"=>2995536636664938496, "baz"=>16831440216388861952}

But when :observe option is used, it always return it like for multi-set

1.9.3p327 (main):004:0> conn.set("foo", "bar", :observe => {:persisted => 1})

{"foo"=>2559248876759744512}