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 the AWS PHP SDK, unless it is first modified to stop passing non-string header values. And thus, that is what this PR does.

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[stobrien89]

Thanks @GrahamCampbell, looks good. Just one thing: could you add a changelog entry by creating .changes/nextrelease and adding a json file by any name with contents in the following format:

[
    {
        "type": "bugfix",
        "category": "Api",
        "description": "<description>"
    }
]

just in the off chance someone hits the new validation paths.

[GrahamCampbell]

Done.