🐛 Bug Report — URLSearchParams body sets incorrect fetch content-type

[nickbabcock]

export default {
    async fetch(request, env, ctx) {
        const r = new Request('https://example.com/', {
            method: 'POST',
            body: new URLSearchParams({ hello: 'world' })
        });
        return new Response(r.headers.get('content-type'))
    },
};

Expected (ref):

application/x-www-form-urlencoded;charset=UTF-8

Actual:

text/plain;charset=UTF-8

This behavior differs from browsers and node.js

Replicated in cloudflare workers playground and in a production workers deployment.