Fix TypeError for GraphqlController#query parameter

What does this MR do and why?

Problem

The application raises a server error when the GraphQL query doesn't have a string type.

Solution

Explicitly convert query to String type.

References

TypeError: Gitlab::RequestForgeryProtection::Co... (#539209 - closed)

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Before After

How to set up and validate locally

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Related to #539209 (closed)

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