Fixed incorrect cast error.

Author: LeeParrishMSFT

libraries/bot-dialogs/src/main/java/com/microsoft/bot/dialogs/prompts/OAuthPrompt.java

@@ -266,7 +266,7 @@ public static CompletableFuture<PromptRecognizerResult<TokenResponse>> recognize
             Object tokenResponseObject = turnContext.getActivity().getValue();
             TokenResponse token = null;
             if (tokenResponseObject != null) {
-                token = (TokenResponse) tokenResponseObject;
+                token = Serialization.getAs(tokenResponseObject, TokenResponse.class);
             }
             result.setSucceeded(true);
             result.setValue(token);