Support applying middleware to both subscription subscribe and resolve functions

[cuzzlor]

Problem defn

When using GraphQL shield to apply rules across the schema, subscription rules are not executed for subscription subscribe operations.

Issue

The applyMiddlewareToField function does not take into account that subscription resolvers can have both subscribe and resolve functions defined.

When applyMiddlewareToField runs, if resolve is defined, subscribe is ignored.

This means that GraphQL shield rules (and other middlewares) are not applied to subscription subscribe operations.

Proposed fix

@leiyangyou posted a patch over here: https://github.com/dimatill/graphql-middleware/issues/561

This patch worked for me, allowing GraphQL shield middleware rules to be executed during the subscribe operation of subscriptions.

All the tests pass with this variant of applyMiddlewareToField, however it is a pretty solid change to the function's structure.

I was not quickly able to work out how to add a test validating the proposed fix.