code: public function set($name, $value = '')
      {
          $name = strtolower($name);
          $this->_headers[$name] = (array) $value;
  
          return $this;
      }
docs: Adds a new header.
  If there is already a header with the same name, it will be replaced.
  @param string $name the name of the header
  @param string $value the value of the header
  @return $this the collection object itself