Encode the request body if Content-Type header is x-www-form-urlencoded

[obulat]

Changes

Fixes #2069
This PR updates the defaultBodySerializer to correctly serialize the body if Content-Type header is set to x-www-form-urlencoded. This is useful for OAuth2 token requests.

How to Review

I'm not sure about the headers: what type are they, and are they normalized?
I added handling for both a plain JS object (headers['Accept-Content']) and a Headers object (headers.get['Accept-Content']), as well as both capitalized and lower-case versions.

Checklist

• Unit tests updated
• docs/ updated (if necessary)
• pnpm run update:examples run (only applicable for openapi-typescript)

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[drwpow]

So these are the headers unique to this request (probably could be named better). Below on line 92, we merge all the headers together. We should probably move that up, adn pass that in here to bodySerializer. That way it takes into account headers that users have set in their createClient() call, and it handles the normalization for you so it’s always a Header type (and you aren’t dealing with what the user passed in)

[drwpow]

This approach looks good to me, and good test! But before we merge, we’ll want to:

1. Use the merged/normalized headers like the comment mentioned
2. Add a patch changeset (see comment)