Support :transit-params

Allow `:transit-params` key that sets content type and encodes as transit -- decodes response body with content type `application/transit+json`.
In an earlier version of transit-cljs, writing a non-map/sequential/set and reading it back resulted in something other than the original value (at least that's what I remember). I thought that was by design for some reason. In the latest version, I no longer see this behavior and will remove.