fix: x-www-form-urlencoded parser keep the BOM

lib/fetch/body.js

@@ -404,7 +404,18 @@ function bodyMixinMethods (instance) {
         // 1. Let entries be the result of parsing bytes.
         let entries
         try {
-          entries = new URLSearchParams(await this.text())
+          let text = ''
+          // application/x-www-form-urlencoded parser will keep the BOM.
+          // https://url.spec.whatwg.org/#concept-urlencoded-parser
+          const textDecoder = new TextDecoder('utf-8', { ignoreBOM: true })
+          for await (const chunk of consumeBody(this[kState].body)) {
+            if (!isUint8Array(chunk)) {
+              throw new TypeError('Expected Uint8Array chunk')
+            }
+            text += textDecoder.decode(chunk, { stream: true })
+          }
+          text += textDecoder.decode()
+          entries = new URLSearchParams(text)
         } catch (err) {
           // istanbul ignore next: Unclear when new URLSearchParams can fail on a string.
           // 2. If entries is failure, then throw a TypeError.

test/fetch/client-fetch.js

@@ -202,6 +202,42 @@ test('urlencoded formData', (t) => {
   })
 })
 
+test('text with BOM', (t) => {
+  t.plan(1)
+
+  const server = createServer((req, res) => {
+    res.setHeader('content-type', 'application/x-www-form-urlencoded')
+    res.end('\uFEFFtest=\uFEFF')
+  })
+  t.teardown(server.close.bind(server))
+
+  server.listen(0, () => {
+    fetch(`http://localhost:${server.address().port}`)
+      .then(res => res.text())
+      .then(text => {
+        t.equal(text, 'test=\uFEFF')
+      })
+  })
+})
+
+test('formData with BOM', (t) => {
+  t.plan(1)
+
+  const server = createServer((req, res) => {
+    res.setHeader('content-type', 'application/x-www-form-urlencoded')
+    res.end('\uFEFFtest=\uFEFF')
+  })
+  t.teardown(server.close.bind(server))
+
+  server.listen(0, () => {
+    fetch(`http://localhost:${server.address().port}`)
+      .then(res => res.formData())
+      .then(formData => {
+        t.equal(formData.get('\uFEFFtest'), '\uFEFF')
+      })
+  })
+})
+
 test('locked blob body', (t) => {
   t.plan(1)