fix(iast): duplicated Flask headers [backport 2.8]

[github-actions]

Backport ad6ac08 from #9014 to 2.8.

Ensure that when tainting the headers of a Flask application, iterating over the headers (i.e., with headers.items()) does not duplicate them.

>> list(request.headers.items())

Now:

[['Host', '0.0.0.0:8000'],
 ['User-Agent', 'python-requests/2.31.0'],
 ['Accept-Encoding', 'gzip, deflate, br'],
 ['Accept', '*/*'],
 ['Connection', 'keep-alive'],
 ['Host', '0.0.0.0:8000'],
 ['User-Agent', 'python-requests/2.31.0'],
 ['Accept-Encoding', 'gzip, deflate, br'],
 ['Accept', '*/*'],
 ['Connection', 'keep-alive']] != [['Host', '0.0.0.0:8000'],
 ['User-Agent', 'python-requests/2.31.0'],
 ['Accept-Encoding', 'gzip, deflate, br'],
 ['Accept', '*/*'],
 ['Connection', 'keep-alive']
]

With this fix:

[
            ["Host", "0.0.0.0:8000"],
            ["User-Agent", "python-requests/2.31.0"],
            ["Accept-Encoding", "gzip, deflate, br"],
            ["Accept", "*/*"],
            ["Connection", "keep-alive"],
]

Introduced in: #8452

Checklist

• Change(s) are motivated and described in the PR description
• Testing strategy is described if automated tests are not included in the PR
• Risks are described (performance impact, potential for breakage, maintainability)
• Change is maintainable (easy to change, telemetry, documentation)
• Library release note guidelines are followed or label changelog/no-changelog is set
• Documentation is included (in-code, generated user docs, public corp docs)
• Backport labels are set (if applicable)
• If this PR changes the public interface, I've notified @DataDog/apm-tees.
• If change touches code that signs or publishes builds or packages, or handles credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.

Reviewer Checklist

• Title is accurate
• All changes are related to the pull request's stated goal
• Description motivates each change
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Change is maintainable (easy to change, telemetry, documentation)
• Release note makes sense to a user of the library
• Author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: backport-9014-to-2.8
Commit report: cb5c59d
Test service: dd-trace-py

✅ 0 Failed, 110963 Passed, 592 Skipped, 2h 7m 29.31s Total duration (3m 53.45s time saved)