fix: add type checking in form data

[trim21]

trafficstars

Summary

ref: https://github.com/encode/httpx/issues/3170

this PR add type checking on application/x-www-form-urlencoded and multipart/form-data content type.

it will throw TypeError on unspported type

Checklist

• [X] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [X] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [ ] I've updated the documentation accordingly.