Content ids for documents bugfix

This is a second attempt at https://github.com/alphagov/whitehall/pull/1869.

There was a bug which meant that the `update_type` option was not honoured by the republishing worker. This was because the options hash passed to the worker is serialised and then deserialised, causing symbol keys to be converted to string keys, and therefore causing the worker to not find an `:update_type` key.

This PR also includes unit tests added by @tekin for `PublishingApiRepublisher`.

I'd like to run another test of this on preview before merging.