interpolate vars correctly in graphql introspection request

[Jackevansevo]

Description

It looks like there was regression with the graphql introspection query code that was preventing pre-request scripts from settings header values in the prepared request.

#704

Using an example here, a pre-script that sets some authentication headers

This data now actually gets sent through to the backend

X-Mul-Nonce: xxxx
X-Mul-Timestamp: xxxx
X-Mul-Signature: xxxxx

So my (previously broken) introspection request now succeeds.

Contribution Checklist:

• The pull request only addresses one issue or adds one feature.
• The pull request does not introduce any breaking changes
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• Create an issue and link to the pull request.

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[helloanoop]

@lohxt1 Can you review this?