vinox, you should override the default file Request.php. copy \app\code\core\Mage\Api2\Model\Request.php to your local directory and add the following code just before end of the getAcceptTypes() Method.
unset($orderedTypes);
$orderedTypes=Array("application/json" => 1);
in other way your getAcceptTypes() method should look like this.
public function getAcceptTypes(){
$qualityToTypes = array();
$orderedTypes = array();
foreach (preg_split('/,\s*/', $this->getHeader('Accept')) as $definition) {
$typeWithQ = explode(';', $definition);
$mimeType = trim(array_shift($typeWithQ));
// check MIME type validity
if (!preg_match('~^([0-9a-z*+\-]+)(?:/([0-9a-z*+\-\.]+))?$~i', $mimeType)) {
continue;
}
$quality = '1.0'; // default value for quality
if ($typeWithQ) {
$qAndValue = explode('=', $typeWithQ[0]);
if (2 == count($qAndValue)) {
$quality = $qAndValue[1];
}
}
$qualityToTypes[$quality][$mimeType] = true;
}
krsort($qualityToTypes);
foreach ($qualityToTypes as $typeList) {
$orderedTypes += $typeList;
}
unset($orderedTypes);
$orderedTypes=Array("application/json" => 1);
return array_keys($orderedTypes);
}