refactor: use our own merge instead of lodash

We have a narrow set of rules for merging objects and lodash's more
aggressive merge may have unintended consequences when the values
being merged are not plain objects.

package.json

@@ -40,8 +40,6 @@
 		"duplexer3": "^0.1.4",
 		"extend": "^3.0.1",
 		"get-stream": "^3.0.0",
-		"lodash.clonedeep": "^4.5.0",
-		"lodash.mergewith": "^4.6.1",
 		"mimic-response": "^1.0.0",
 		"p-cancelable": "^0.5.0",
 		"to-readable-stream": "^1.0.0",

readme.md

@@ -406,12 +406,14 @@ client.get('/demo');
 
 #### got.assignOptions(parentOptions, newOptions)
 
-Extends parent options. Options are deeply merged to a new object as follows:
-
-- If either value is `null`, a primitive, or an `Array`, the result is the newOptions value.
-- If the parentOptions value is an instance of `URL`, the resulting value is the result of `new URL(new, parent)`.
-- If the newOptions value is `undefined` the key is removed.
-- If the newOptions value is an `Object`, its values are merged recursively.
+Extends parent options. Options are deeply merged to a new object. The value of each key is determined as follows:
+
+- If the newOptions value is `undefined` the key is omitted from the result.
+- If the parentOptions value is an instance of `URL` and the newOptions value is a `string` or `URL`, a new URL instance is created with the p `new URL(new, parent)`.
+- If the newOptions value is a plain `Object`
+  - If the parentOptions value is a plain `Object`, both values are merged recursively into a new `Object`.
+  - Otherwise, only the newOptions value is merged recurively into a new `Object`.
+- Otherwise, the newOptions value is assigned to the key.
 
 Avoid using [object spread](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax#Spread_in_object_literals) as it doesn't work recursively:
 

source/assign-options.js

@@ -1,36 +1,30 @@
 const url = require('url');
-const mergeWith = require('lodash.mergewith');
-const cloneDeep = require('lodash.clonedeep');
 const is = require('@sindresorhus/is');
 
 module.exports = (defaults, options = {}) => {
-	return mergeWith(
-		{opts: cloneDeep(defaults)},
-		{opts: options},
-		customizer
-	).opts;
+	return merge({}, defaults, options);
 };
 
-function customizer(objValue, srcValue) {
-	if (is.array(srcValue) || is.array(objValue)) {
-		return cloneDeep(srcValue);
-	}
-	if (objValue instanceof url.URL) {
-		return new url.URL(srcValue, objValue);
-	}
-	if (![objValue, srcValue].some(is.array) && [objValue, srcValue].every(is.object)) {
-		// When both args are non-array objects, delete keys for which the source
-		// value is undefined (null is a significant value places, e.g. `encoding`).
-		const deleteKeys = [];
-		for (const key in srcValue) {
-			if (is.undefined(srcValue[key])) {
-				deleteKeys.push(key);
+function merge(target = {}, ...sources) {
+	for (const source of sources) {
+		for (const [key, sourceValue] of Object.entries(source)) {
+			const targetValue = target[key];
+			if (is.undefined(sourceValue)) {
+				delete target[key];
+			} else if (is.urlInstance(targetValue) && (
+				is.urlInstance(sourceValue) || is.string(sourceValue)
+			)) {
+				target[key] = new url.URL(sourceValue, targetValue);
+			} else if (is.plainObject(sourceValue)) {
+				if (is.plainObject(targetValue)) {
+					target[key] = merge({}, targetValue, sourceValue);
+				} else {
+					target[key] = merge({}, sourceValue);
+				}
+			} else {
+				target[key] = sourceValue;
 			}
 		}
-		const result = mergeWith(objValue, srcValue, customizer);
-		for (const key of deleteKeys) {
-			delete result[key];
-		}
-		return result;
 	}
+	return target;
 }

test/create.js

@@ -77,11 +77,11 @@ test('custom headers (extend)', async t => {
 	t.is(headers.unicorn, 'rainbow');
 });
 
-test('extend overwrites arrays with copy', t => {
+test('extend overwrites arrays', t => {
 	const statusCodes = [408];
 	const a = got.extend({retry: {statusCodes}});
 	t.deepEqual(a.defaults.options.retry.statusCodes, statusCodes);
-	t.not(a.defaults.options.retry.statusCodes, statusCodes);
+	t.is(a.defaults.options.retry.statusCodes, statusCodes);
 });
 
 test('extend removes object values set to undefined', t => {