fix: not passing config.init to fetch

Fixes a bug causing `config.init` not to be passed as the `init` parameter to `fetch`.

src/retrieve.test.ts

@@ -78,6 +78,52 @@ describe('retrieve', () => {
 		})
 
 		describe('init', () => {
+			test('config.init parameters are passed to fetch', async () => {
+				vi.spyOn(global, 'fetch').mockImplementation((...fetchParams: OriginalFetchParams) => {
+					assertInitEquality(fetchParams[1], {
+						body: 'body',
+						cache: 'default',
+						credentials: 'same-origin',
+						headers: new Headers({
+							'x-test-header': 'header-value',
+							'x-requested-with': 'XMLHttpRequest',
+						}),
+						integrity: 'hash',
+						keepalive: true,
+						method: 'POST',
+						mode: 'same-origin',
+						redirect: 'follow',
+						referrer: 'ref',
+						signal: null,
+						window: null,
+					})
+
+					return Promise.resolve(new Response('OK'))
+				})
+
+				await retrieve({
+					url: 'http://example.org',
+					init: {
+						body: 'body',
+						cache: 'default',
+						credentials: 'same-origin',
+						headers: {
+							'x-test-header': 'header-value',
+						},
+						integrity: 'hash',
+						keepalive: true,
+						method: 'POST',
+						mode: 'same-origin',
+						redirect: 'follow',
+						referrer: 'ref',
+						signal: null,
+						window: null,
+					},
+				})
+
+				expect(global.fetch).toHaveBeenCalled()
+			})
+
 			describe('method', () => {
 				test.each([
 					[

src/retrieve.ts

@@ -363,7 +363,7 @@ function createUrl(config: RetrieveConfig): URL {
 
 function createInit(config: RetrieveConfig): RequestInit {
 	const originalInit: RequestInit = config.init ?? {}
-	const init: RequestInit = {}
+	const init: RequestInit = { ...originalInit }
 
 	// Process request method
 	init.method = (originalInit.method ?? 'GET').toUpperCase()