使用OAuth-Signpost和Apache httpcomponent对POST请求进行签名的正确方法是什么?

I'm currently using the OAuth-Signpost Java library to sign requests sent from a client to a server which implements OAuth authentication. When making GET requests (using HttpURLConnection) everything works fine: requests are signed, parameters are included and signatures match in destination. However, it doesn't seem to work with POST requests. I'm aware of the issues that may come up when signing POST using HttpURLConnection, so I moved to the Apache HttpComponents library for these requests. The parameters I send in the following example are plain strings and a XML-like string ('rxml'). My code goes as follows:I'm currently using the OAuth-Signpost Java lib