Fix handling of custom response_wrapper in test.Client

[jace]

This patch replaces the incorrect isinstance check with issubclass in how test.Client processes the response_wrapper argument. Without this patch, supplying a subclass of werkzeug.test.TestResponse and flask.wrappers.Response will fail with TypeError: Cannot create a consistent method resolution.

• fixes test.Client is incorrectly using isinstance check instead of issubclass #2831

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[pgjones]

Thank you