fix(): subscription.subscribe() not copied

[igrek8]

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Methods decorated with @Subscription are not copied into the resolvers. As a result, AsyncIterator is not returned causing Subscription field must return Async Iterable. Received: undefined

Issue Number: 663

What is the new behavior?

References to methods are copied accordingly. Subscribers subscribe to AsyncIterators and receive updates.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Copying subscribe() does not affect any resolvers.

[igrek8]

#663

[kamilmysliwiec]

Thank you!