Revisited Tenancy#as_json specs

added test for agreement_reissued_for_same_landlord_and_tenant
Added more unit tests to Tenancy#as_json
Removed left over setter & getter in Tenancy

They were there in order to get certain specs passing.
But they should not be there any longer.
Merge pull request #163 from ministryofjustice/as_json

As json