Fix incorrect credentials handling for client_credentials grant type

Fixes #869

Author: Kieun

src/templates/security-scheme-template.js

@@ -105,8 +105,7 @@ async function fetchAccessToken(tokenUrl, clientId, clientSecret, redirectUrl, g
     urlFormParams.append('code_verifier', codeVerifier); // for PKCE
   }
   if (sendClientSecretIn === 'header') {
-    // headers.set('Authorization', `Basic ${btoa(`${clientId}:${clientSecret}`)}`);
-    headers.set('Authorization', `Basic ${Buffer.from(`${username}:${password}`, 'utf8').toString('base64')}`);
+    headers.set('Authorization', `Basic ${Buffer.from(`${clientId}:${clientSecret}`, 'utf8').toString('base64')}`);
   } else {
     urlFormParams.append('client_id', clientId);
     urlFormParams.append('client_secret', clientSecret);