Submit a Post to symfony FosRestBundle - rest

Question

As I try to use a REST API, I setted up a simple controller which should only return my POST Vars as a xml response.
Unfortunately, it won't return it. I guess I have some probs by getting the data from the request.
This is my Action inside the Controller. GET works fine, but POST won't do.
public function postAuthAction (Request $request)
{
#$request = $this->get('request');
$username = $request->request->get('Username');
$password = $request->request->get('Password');
$data=array("user"=>$username,"password"=>$password);
return $data;
}
It only returns me:
<result>
<entry>
<entry xsi:nil="true"/>
<entry xsi:nil="true"/>
</entry>
</result>
Which isn't that bad for the reason that it returns anything at all ;)
I extpected something like
<result>
<user>POST Username</user>
<password>POST Password</password>
</result>
What am I doing wrong? IO don't get it.
EDIT: my POST
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Login>
<Username>abc</Username>
<Password>pass12</Password>
</Login>