CreateSubscriptionStream doesn't send 'connection_init'

[chelliwell]

[Ref subscriptions-api branch]
Whilst there is a definition of this message (GQL_CONNECTION_INIT), it doesn't seem to be used at all. Shouldn't this be being sent by GraphQLHttpSubscriptionHelpers.CreateSubscriptionStream prior to the subscription request?

I think the omission of this is why Apollo server is not seeing its subscriptions 'onConnect' event (it does if subscribing with Playground instead).
In fact, the code doesn't seem to [yet? work-in-progress?] make use of any of the GQL_CONNECTION_ strings

[rose-a]

fixed in #152