code: public function setDefault($name, $value)
      {
          $name = strtolower($name);
          if (empty($this->_headers[$name])) {
              $this->_headers[$name][] = $value;
          }
  
          return $this;
      }
docs: Sets a new header only if it does not exist yet.
  If there is already a header with the same name, the new one will be ignored.
  @param string $name the name of the header
  @param string $value the value of the header
  @return $this the collection object itself