fix: cast generated header values to strings

[GrahamCampbell]

Hi there. The Guzzle maintainer here again. I am working on the next major version of Guzzle with a focus on security hardening and reducing security-foot guns. As part of those changes, the next major version will disallow non-string header values. I will be triggering deprecations soon on the current major version series, which will start to impact this project, unless it is first modified to stop passing non-string header values. And thus, that is what this PR does.

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FYI, there is a similar PR open on a sister repo at googleapis/google-cloud-php#9208.

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[bshaffer]

We need to update symfony/dom-crawler to version 5 or higher to resolve the vulnerability which is failing the tests

[GrahamCampbell]

I think that would be out of scope for this PR.

[GrahamCampbell]

#2718