Prefer #is_a? over === for readability and style consistency

Co-authored-by: Santiago Rodriguez <[email protected]>

lib/webauthn/authenticator_assertion_response.rb

@@ -14,7 +14,7 @@ def self.from_client(response, relying_party: WebAuthn.configuration.relying_par
       encoder = relying_party.encoder
 
       user_handle =
-        if response["userHandle"] && String === response["userHandle"]
+        if response["userHandle"] && response["userHandle"].is_a? String
           encoder.decode(response["userHandle"])
         end