add keep alive option to fetch standard transport (#787)

Author: silesky

.changeset/orange-pumpkins-attack.md

@@ -0,0 +1,5 @@
+---
+'@segment/analytics-next': minor
+---
+
+Add keep-alive header to fetch for standard transport.

packages/browser/src/plugins/segmentio/fetch-dispatcher.ts

@@ -5,6 +5,7 @@ export type Dispatcher = (url: string, body: object) => Promise<unknown>
 export default function (): { dispatch: Dispatcher } {
   function dispatch(url: string, body: object): Promise<unknown> {
     return fetch(url, {
+      keepalive: true,
       headers: { 'Content-Type': 'text/plain' },
       method: 'post',
       body: JSON.stringify(body),