fix: fetchEvent -> HeaderProxy checks for null instead of undefined (#1919)

Author: huseeiin

.changeset/open-ducks-flash.md

@@ -0,0 +1,5 @@
+---
+"@solidjs/start": patch
+---
+
+fix: check for `null` instad of `undefined` in `HeaderProxy`

packages/start/src/server/fetchEvent.ts

@@ -53,7 +53,7 @@ class HeaderProxy {
 		return Array.isArray(h) ? h.join(", ") : (h as string) || null;
 	}
 	has(key: string) {
-		return this.get(key) !== undefined;
+		return this.get(key) !== null;
 	}
 	set(key: string, value: string) {
 		return setResponseHeader(this.event, key, value);