Exception in OnSendingHeaders not handled resulting in request hangs

[Praburaj]

Currently exceptions in OnSendingHeaders are not handled in Kestrel resulting in request hangs when an exception is thrown.

I ran into this issue with aspnet/Security#28. Kestrel in this case is resulting in a request hang.

[davidfowl]

/cc @muratg I moved this to beta6

[halter73]

@muratg I just submitted a PR out that fixes this, but I don't think it's worth pushing through ask mode to get this in beta 6. Does punting this to beta 7 sound OK?