code: private function _selectContentTypeHeader($contentType)
      {
          if (count($contentType) === 0 || (count($contentType) === 1 && $contentType[0] === '')) {
              return 'application/json';
          } elseif (preg_grep("/application\/json/i", $contentType)) {
              return 'application/json';
          } else {
              return implode(',', $contentType);
          }
      }
docs: /*
  Return the content type based on an array of content-type provided
  
  @param string[] $contentType Array fo content-type
  
  @return string Content-Type (e.g. application/json)