Twitter API status update always returns "Incorrect signature"

I'm using Signpost as OAuth implementation for posting to Twitter. And implemented the GoogleAppEngineOAuthConsumer and GoogleAppEngineOAuthProvider classes, but since they're pretty trivial, so I'm not providing their sources here (yet).

Here's my authentication part, which seems to work just fine.

LoginServlet.java:

// fetching the request token
OAuthConsumer consumer = new GoogleAppEngineOAuthConsumer(CONSUMER_KEY, CONSUMER_SECRET);
OAuthProvider provider = new GoogleAppEngineOAuthProvider(REQUEST_TOKEN_URL, ACCESS_TOKEN_URL, AUTHORIZATION_URL);
String redirectUrl = provider.retrieveRequestToken(consumer, CALLBACK_URL);
// cache the request token and request token secret
response.sendRedirect(redirectUrl);

CallbackServlet.java

// fetching the access token
String verifier = (String) req.getParameter("oauth_verifier");
// retrieve request token and request token secret from cache
OAuthConsumer consumer = new GoogleAppEngineOAuthConsumer(CONSUMER_KEY, CONSUMER_SECRET);
OAuthProvider provider = new GoogleAppEngineOAuthProvider(REQUEST_TOKEN_URL, 
consumer.setTokenWithSecret(token, tokenSecret);
provider.setOAuth10a(true);
provider.retrieveAccessToken(consumer, verifier);
// store access token and access token secret

And here's the 开发者_如何学运维actual problematic part.

TweetServlet.java

OAuthConsumer consumer = new GoogleAppEngineOAuthConsumer(CONSUMER_KEY, CONSUMER_SECRET);
// retrieve access token and access token secret from storage
consumer.setTokenWithSecret(accessToken, accessTokenSecret);
final HTTPRequest updateStatus = new HTTPRequest(new URL("http://api.twitter.com/1/statuses/update.json"), HTTPMethod.POST);
updateStatus.setPayload(("status=" + URLEncoder.encode(message, "UTF-8")).getBytes());
consumer.sign(updateStatus);
logger.debug(new String(URLFetchServiceFactory.getURLFetchService().fetch(updateStatus).getContent()));

Each and every time it results: {"request":"/1/statuses/update.json","error":"Incorrect signature"}.

I was able to solve this by myself. The problem was that I wasn't setting a Content-Type header to the request, so the signing didn't sign the parameters and it resulted the invalid signature. Once I set it to application/x-www-form-urlencoded it started working.

final HTTPRequest updateStatus = new HTTPRequest(new URL("http://api.twitter.com/1/statuses/update.json"), HTTPMethod.POST);
updateStatus.addHeader(new HTTPHeader("Content-Type", "application/x-www-form-urlencoded"));