Remove content-type header check for FormData body (#208)

index.js

@@ -46,7 +46,6 @@ for (const property of globalProperties) {
 const isObject = value => value !== null && typeof value === 'object';
 const supportsAbortController = typeof globals.AbortController === 'function';
 const supportsStreams = typeof globals.ReadableStream === 'function';
-const supportsFormData = typeof globals.FormData === 'function';
 
 const deepMerge = (...sources) => {
 	let returnValue = {};
@@ -240,11 +239,7 @@ class Ky {
 		if (this._options.searchParams) {
 			const url = new URL(this.request.url);
 			url.search = new URLSearchParams(this._options.searchParams);
-			this.request = new globals.Request(url, this.request);
-		}
-
-		if (((supportsFormData && this._options.body instanceof globals.FormData) || this._options.body instanceof URLSearchParams) && this.request.headers.has('content-type')) {
-			throw new Error(`The \`content-type\` header cannot be used with a ${this._options.body.constructor.name} body. It will be set automatically.`);
+			this.request = new globals.Request(new globals.Request(url, this.request), this._options);
 		}
 
 		if (this._options.json) {

readme.md

@@ -416,7 +416,7 @@ import ky from 'ky';
 
 ### Sending Form Data
 
-Sending form data in Ky is identical to `fetch`. Just pass a [`FormData`](https://developer.mozilla.org/en-US/docs/Web/API/FormData) instance to the `body` option. The `Content-Type` header will be automatically set to `multipart/form-data`. Setting it manually will result in an error.
+Sending form data in Ky is identical to `fetch`. Just pass a [`FormData`](https://developer.mozilla.org/en-US/docs/Web/API/FormData) instance to the `body` option. The `Content-Type` header will be automatically set to `multipart/form-data`.
 
 ```js
 import ky from 'ky';

test/browser.js

@@ -1,3 +1,5 @@
+import util from 'util';
+import body from 'body';
 import {serial as test} from 'ava';
 import createTestServer from 'create-test-server';
 import withPage from './helpers/with-page';
@@ -176,3 +178,61 @@ test('throws if does not support ReadableStream', withPage, async (t, page) => {
 
 	await server.close();
 });
+
+test('FormData with searchParams', withPage, async (t, page) => {
+	const server = await createTestServer();
+	server.get('/', (request, response) => {
+		response.end();
+	});
+	server.post('/', async (request, response) => {
+		const pBody = util.promisify(body);
+		const requestBody = await pBody(request);
+
+		t.regex(requestBody, /bubblegum pie/);
+		t.deepEqual(request.query, {foo: '1'});
+
+		response.end();
+	});
+
+	await page.goto(server.url);
+	await page.addScriptTag({path: './umd.js'});
+	await page.evaluate(url => {
+		window.ky = window.ky.default;
+		const formData = new window.FormData();
+		formData.append('file', new window.File(['bubblegum pie'], 'my-file'));
+		return window.ky(url, {
+			method: 'post',
+			searchParams: 'foo=1',
+			body: formData
+		});
+	}, server.url);
+
+	await server.close();
+});
+
+test('headers are preserved when input is a Request and there are searchParams in the options', withPage, async (t, page) => {
+	const server = await createTestServer();
+	server.get('/', (request, response) => {
+		response.end();
+	});
+	server.get('/test', (request, response) => {
+		t.is(request.headers['content-type'], 'text/css');
+		t.deepEqual(request.query, {foo: '1'});
+		response.end();
+	});
+
+	await page.goto(server.url);
+	await page.addScriptTag({path: './umd.js'});
+
+	await page.evaluate(url => {
+		window.ky = window.ky.default;
+		const request = new window.Request(url + '/test', {
+			headers: {'content-type': 'text/css'}
+		});
+		return window.ky(request, {
+			searchParams: 'foo=1'
+		}).text();
+	}, server.url);
+
+	await server.close();
+});

test/main.js

@@ -485,15 +485,6 @@ test('throws AbortError when aborted by user', async t => {
 	await t.throwsAsync(response, {name: 'AbortError'});
 });
 
-test('throws when using FormData with `content-type` header', t => {
-	t.throws(() => ky.post('https://example.com', {
-		body: new URLSearchParams(),
-		headers: {
-			'content-type': ''
-		}
-	}), 'The `content-type` header cannot be used with a URLSearchParams body. It will be set automatically.');
-});
-
 test('supports Request instance as input', async t => {
 	const server = await createTestServer();
 	const inputRequest = new Request(server.url, {method: 'POST'});