fix(fetch): allow third-party FormData/Blob bodies

`webidl.converters.XMLHttpRequestBodyInit` rejected third-party
`FormData`/`Blob`s, meaning they were passed through
`webidl.converters.DOMString`. This meant `Request`/`Response`s
constructed with third-party `FormData`/`Blob` bodies ended up with a
plain-text `[object FormData]`/`[object Blob]` body.

lib/fetch/response.js

@@ -519,7 +519,7 @@ webidl.converters.XMLHttpRequestBodyInit = function (V) {
   }
 
   if (isBlobLike(V)) {
-    return webidl.converters.Blob(V)
+    return webidl.converters.Blob(V, { strict: false })
   }
 
   if (
@@ -530,8 +530,8 @@ webidl.converters.XMLHttpRequestBodyInit = function (V) {
     return webidl.converters.BufferSource(V)
   }
 
-  if (V instanceof FormData) {
-    return webidl.converters.FormData(V)
+  if (util.isFormDataLike(V)) {
+    return webidl.converters.FormData(V, { strict: false })
   }
 
   if (V instanceof URLSearchParams) {

test/fetch/request.js

@@ -9,6 +9,10 @@ const {
   fetch
 } = require('../../')
 const { kState } = require('../../lib/fetch/symbols.js')
+const {
+  Blob: ThirdPartyBlob,
+  FormData: ThirdPartyFormData
+} = require('formdata-node')
 const hasSignalReason = !!~process.version.localeCompare('v16.14.0', undefined, { numeric: true })
 
 test('arg validation', async (t) => {
@@ -435,3 +439,23 @@ test('RequestInit.signal option', async (t) => {
     signal: false
   }), TypeError)
 })
+
+test('constructing Request with third party Blob body', async (t) => {
+  const blob = new ThirdPartyBlob(['text'])
+  const req = new Request('http://asd', {
+    method: 'POST',
+    body: blob
+  })
+  t.equal(await req.text(), 'text')
+})
+test('constructing Request with third party FormData body', async (t) => {
+  const form = new ThirdPartyFormData()
+  form.set('key', 'value')
+  const req = new Request('http://asd', {
+    method: 'POST',
+    body: form
+  })
+  const contentType = req.headers.get('content-type').split('=')
+  t.equal(contentType[0], 'multipart/form-data; boundary')
+  t.ok((await req.text()).startsWith(`--${contentType[1]}`))
+})

test/fetch/response.js

@@ -5,6 +5,10 @@ const {
   Response
 } = require('../../')
 const { ReadableStream } = require('stream/web')
+const {
+  Blob: ThirdPartyBlob,
+  FormData: ThirdPartyFormData
+} = require('formdata-node')
 
 test('arg validation', async (t) => {
   // constructor
@@ -230,3 +234,17 @@ test('constructing a Response with a ReadableStream body', async (t) => {
 
   t.end()
 })
+
+test('constructing Response with third party Blob body', async (t) => {
+  const blob = new ThirdPartyBlob(['text'])
+  const res = new Response(blob)
+  t.equal(await res.text(), 'text')
+})
+test('constructing Response with third party FormData body', async (t) => {
+  const form = new ThirdPartyFormData()
+  form.set('key', 'value')
+  const res = new Response(form)
+  const contentType = res.headers.get('content-type').split('=')
+  t.equal(contentType[0], 'multipart/form-data; boundary')
+  t.ok((await res.text()).startsWith(`--${contentType[1]}`))
+})