Setting an identical value don't constitute a change

Telling a Trackie instance `x = Trackie.new(a: "foo")` that `x[:a] = "foo"` would mark it as `changed?`. Thus, when using this Trackie to send an update request, this changed/unchanged attribute would be sent along as an update as well, causing the request to fail if the attribute isn't allowed by Zendesk.

I'm not entirely sure why/where/when the Trackie is being set with identical values, but it is definitely causing problems :-)

cc/ @jlauemoeller, @jespr, @steved555