Fix response_mode=form_post with Starlette client

[azmeuk]

What kind of change does this PR introduce?

This makes the Starlette client handle the response_mode=form_post as suggested in #793. The behavior mimicks the Flask client.

Checklist

• The commits follow the conventional commits specification.
• You ran the linters with prek.
• You wrote unit test to demonstrate the bug you are fixing, or to stress the feature you are bringing.
• You reached 100% of code coverage on the code you edited, without abusive use of pragma: no cover

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