Update as_json to return string keys

lib/action_pack/web_authn/public_key_credential/request_options.rb

@@ -35,12 +35,14 @@ class ActionPack::WebAuthn::PublicKeyCredential::RequestOptions < ActionPack::We
   # Returns a Hash suitable for JSON serialization and passing to the
   # WebAuthn JavaScript API.
   def as_json(options = {})
-    {
+    json = {
       challenge: challenge,
       rpId: relying_party.id,
       allowCredentials: credentials.map { |credential| allow_credential_json(credential) },
       userVerification: user_verification.to_s
-    }.as_json(options)
+    }
+
+    json.as_json(options)
   end
 
   private