Drop support for body being an Array when form: true

`new URLSearchParams()` doesn't support this and it's a weird use-case. If you need support for this, just don't set `form: true` and handle stringifying yourself.

index.js

@@ -6,7 +6,6 @@ const https = require('https');
 const {PassThrough, Transform} = require('stream');
 const urlLib = require('url');
 const fs = require('fs');
-const querystring = require('querystring');
 const URLSearchParamsGlobal = typeof URLSearchParams === 'undefined' ? require('url').URLSearchParams : URLSearchParams; // TODO: Use the `URL` global when targeting Node.js 10
 const extend = require('extend');
 const CacheableRequest = require('cacheable-request');
@@ -580,18 +579,21 @@ function normalizeArguments(url, opts) {
 			throw new TypeError('The `body` option must be a stream.Readable, string or Buffer');
 		}
 
-		const canBodyBeStringified = is.plainObject(body) || is.array(body);
-		if ((opts.form || opts.json) && !canBodyBeStringified) {
-			throw new TypeError('The `body` option must be a plain Object or Array when the `form` or `json` option is used');
+		if (opts.json && !(is.plainObject(body) || is.array(body))) {
+			throw new TypeError('The `body` option must be a plain Object or Array when the `json` option is used');
+		}
+
+		if (opts.form && !is.plainObject(body)) {
+			throw new TypeError('The `body` option must be a plain Object when the `form` option is used');
 		}
 
 		if (isFormData(body)) {
 			// Special case for https://github.com/form-data/form-data
 			headers['content-type'] = headers['content-type'] || `multipart/form-data; boundary=${body.getBoundary()}`;
-		} else if (opts.form && canBodyBeStringified) {
+		} else if (opts.form && is.plainObject(body)) {
 			headers['content-type'] = headers['content-type'] || 'application/x-www-form-urlencoded';
-			opts.body = querystring.stringify(body);
-		} else if (opts.json && canBodyBeStringified) {
+			opts.body = (new URLSearchParamsGlobal(body)).toString();
+		} else if (opts.json && (is.plainObject(body) || is.array(body))) {
 			headers['content-type'] = headers['content-type'] || 'application/json';
 			opts.body = JSON.stringify(body);
 		}

readme.md

@@ -149,7 +149,7 @@ Default: `false`
 
 If set to `true` and `Content-Type` header is not set, it will be set to `application/x-www-form-urlencoded`.
 
-`body` must be a plain object or array and will be stringified.
+`body` must be a plain object. It will be converted to a query string using [`(new URLSearchParams(object)).toString()`](https://nodejs.org/api/url.html#url_constructor_new_urlsearchparams_obj).
 
 ###### json
 

test/error.js

@@ -59,12 +59,12 @@ test('options.body error message', async t => {
 
 test('options.body json error message', async t => {
 	const err = await t.throws(got(s.url, {body: Buffer.from('test'), json: true}));
-	t.regex(err.message, /The `body` option must be a plain Object or Array when the `form` or `json` option is used/);
+	t.regex(err.message, /The `body` option must be a plain Object or Array when the `json` option is used/);
 });
 
 test('options.body form error message', async t => {
 	const err = await t.throws(got(s.url, {body: Buffer.from('test'), form: true}));
-	t.regex(err.message, /The `body` option must be a plain Object or Array when the `form` or `json` option is used/);
+	t.regex(err.message, /The `body` option must be a plain Object when the `form` option is used/);
 });
 
 test('default status message', async t => {

test/post.js

@@ -53,12 +53,11 @@ test('sends plain objects as forms', async t => {
 	t.is(body, 'such=wow');
 });
 
-test('sends arrays as forms', async t => {
-	const {body} = await got(s.url, {
+test('does NOT support sending arrays as forms', async t => {
+	await t.throws(got(s.url, {
 		body: ['such', 'wow'],
 		form: true
-	});
-	t.is(body, '0=such&1=wow');
+	}), TypeError);
 });
 
 test('sends plain objects as JSON', async t => {