This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Seems to me that it doesn't matter whether the data you are sending happens to be XML. You have a question about Apache HttpClient (it is Apache, right? You didn't mention that...) so let's not hide it in the XML forum.