add external_id test for BlockingRouter
add external_id test for BlockingRouter

peterayeni/rapidsms,catalpainternational/rapidsms,caktus/rapidsms,eHealthAfrica/rapidsms,lsgunth/rapidsms,ehealthafrica-ci/rapidsms,eHealthAfrica/rapidsms,ehealthafrica-ci/rapidsms,peterayeni/rapidsms,lsgunth/rapidsms,catalpainternational/rapidsms,eHealthAfrica/rapidsms,caktus/rapidsms,catalpainternational/rapidsms,lsgunth/rapidsms,catalpainternational/rapidsms,lsgunth/rapidsms,ehealthafrica-ci/rapidsms,peterayeni/rapidsms,caktus/rapidsms,peterayeni/rapidsms