fix: make Authorization header check case-insensitive

- Fix issue #1043 where custom Authorization headers were checked case-sensitively
- HTTP headers should be case-insensitive according to RFC standards
- Replace exact key match with case-insensitive check using Object.keys().some()
- This allows headers like 'authorization', 'Authorization', 'AUTHORIZATION' to work correctly

Author: Louis454545

src/SupabaseClient.ts

@@ -315,7 +315,9 @@ export default class SupabaseClient<
       fetch,
       // auth checks if there is a custom authorizaiton header using this flag
       // so it knows whether to return an error when getUser is called with no session
-      hasCustomAuthorizationHeader: 'Authorization' in this.headers,
+      hasCustomAuthorizationHeader: Object.keys(this.headers).some(
+        (key) => key.toLowerCase() === 'authorization'
+      ),
     })
   }