fix: check the actual GraphQLDirective instance in isSpecifiedDirective

[ardatan]

Related ardatan/graphql-tools#7716

Fixes the bug when a GraphQLDirective instance is passed with the same name as specified directives, isSpecifiedDirective returns true unexpectedly.

[vercel]

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[yaacovCR]

The spec assumes that all directives with the names of specified directives are the actual specified directives, but it seems to me we should be more permissive as possible within the implementation, especially to enable custom/experimental forms of the different directives.

In terms of future-proofing this, is the contention that we only need one function describing the actual specified directives, or might we need two different functions, one version that retains the current behavior (returning true for custom directives that override the specified ones) and one with the "fixed" behavior?

And basically we have the parallel question with isSpecifiedScalar...