fix: add type checking in form data

[trim21]

Summary

ref: #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

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• 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.