code: public function addHeader(string $header, $value):void
  	{
  	    if (is_iterable($value) || is_array($value)) {
  	        $iterable = $value;
  	        $value = [];
  	        foreach ($iterable as $key => $entry) {
                  $value[$key] = (string)$entry;
              }
          }
          else {
  	        $value = (string)$value;
          }
  
  		$this->headers[] = [$header, $value];
  	}
docs: Adds one header.
  
  @param string $header
  @param string|array|iterable $value