Set content-type for webhooks post request.
Set content-type for webhooks post request.

Also add missing call to db_session.commit() on stopping a webhook.

wakermahmud/sync-engine,ErinCall/sync-engine,closeio/nylas,PriviPK/privipk-sync-engine,Eagles2F/sync-engine,gale320/sync-engine,wakermahmud/sync-engine,ErinCall/sync-engine,PriviPK/privipk-sync-engine,rmasters/inbox,EthanBlackburn/sync-engine,EthanBlackburn/sync-engine,jobscore/sync-engine,ErinCall/sync-engine,rmasters/inbox,wakermahmud/sync-engine,nylas/sync-engine,ErinCall/sync-engine,Eagles2F/sync-engine,Eagles2F/sync-engine,wakermahmud/sync-engine,closeio/nylas,ErinCall/sync-engine,wakermahmud/sync-engine,nylas/sync-engine,gale320/sync-engine,closeio/nylas,PriviPK/privipk-sync-engine,jobscore/sync-engine,gale320/sync-engine,PriviPK/privipk-sync-engine,EthanBlackburn/sync-engine,rmasters/inbox,EthanBlackburn/sync-engine,PriviPK/privipk-sync-engine,rmasters/inbox,jobscore/sync-engine,nylas/sync-engine,gale320/sync-engine,EthanBlackburn/sync-engine,nylas/sync-engine,gale320/sync-engine,closeio/nylas,Eagles2F/sync-engine,jobscore/sync-engine,Eagles2F/sync-engine