Respond to JSONP send requests with the correct MIME type

#3110
Rewrite ProcessStartRequests to not use async/await to avoid deadlocks

- Deadlocks would occur in the old async/await code if someone attached
  to the PreSendRequestHeaders event.

#3116
Attach to the PreSendRequestHeaders event for only one test
Bump the ConnectionsDisconnected PC even when OnDisconnected throws
Removed the old OnDisonnected overload

- Removing the old overload is preferable to changing its behavior

#3115
Merge branch 'release' into dev