Store dialog_id from route hdr on in dialog requests

Hi Andy,

Please can you review this fix to store off the dialog_id from the top route header on in dialog requests in the SproutletAppServerShimTsx?

This fixes https://github.com/Metaswitch/sprout/issues/813.

There don't appear to be any UTs for this file at the moment - do you know why? Either way, I've live tested by running some calls with memento.

Thanks,
Graeme