Fix(beckend): html content-type detection on signedGet (misskey-dev#14424)

* fix(backend): contenttype detection of html in signedGet

* code style

* fix by review

Author: tai-cha

packages/backend/src/core/activitypub/ApRequestService.ts

@@ -204,8 +204,8 @@ export class ApRequestService {
 
 		//#region リクエスト先がhtmlかつactivity+jsonへのalternate linkタグがあるとき
 		const contentType = res.headers.get('content-type');
-		
-		if ((contentType === 'text/html' || contentType?.startsWith('text/html;')) && _followAlternate === true) {
+
+		if ((contentType ?? '').split(';')[0].trimEnd().toLowerCase() === 'text/html' && _followAlternate === true) {
 			const html = await res.text();
 			const window = new Window();
 			const document = window.document;