Subscriptions sent before connack are concatenated incorrectly

[boxmein]

https://github.com/Qminder/javascript-api/blob/master/src/services/GraphQLService.ts#L290

Subscriptions sent before GQL_CONNECTION_ACK are stored and resumed when CONNECTION_ACK is achieved. Also, whenever connection drops and reestablishes, the same code is used to reestablish subscriptions.

However, this code also adds subscription { ... } around the query, which was actually removed from most of the code in #310 .

This place should remove them as well.