Zeus generated file is incorrect when targeting Node and Graphql-ws subscriptions

[gthau]

When targeting Node (--n) and Graphql-ws subscriptions (--subscriptions graphql-ws), the generated file has 2 issues that cause compile time and run time errors:

1. tsc error: using global Headers type definition instead of node-fetch Headers type definition
2. runtime error: not passing the webSocketImpl to the createClient function

This PR fixes both issues: #390 (there's an open question about the apiSubscription function)