Don't skip opId === 0

[cvle]

I noticed that the first subscription is never unsubscribed.
Turns out that the first operation id is zero and that the if (opId) {
check is the culprit.

Fixes: #180

TODO:

• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change

[apollo-cla]

@cvle: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[NeoPhi]

Great catch @cvle. Thank you for the PR!

[cvle]

@NeoPhi Awesome! Can you make a release? This was the source for some warnings on our side. Would be great to get rid of them :-)

[NeoPhi]

@dotansimha or @Urigo See above.

[cvle]

@NeoPhi I saw that recently the typings of operation id was changed to a string. I didn't know. The typings should be adapted and the check should be changed to sth like: if (opId || opId === 0).

[cvle]

https://github.com/apollographql/graphql-subscriptions still has number typings for opIds.

[dotansimha]

@cvle where is the number for opIds?

[Urigo]

thanks @cvle ! released in 0.7.3