/**
* Return an instance with the specified header appended with the given value.
*
* Existing values for the specified header will be maintained. The new
* value(s) will be appended to the existing list. If the header did not
* exist previously, it will be added.
*
* This method MUST be implemented in such a way as to retain the
* immutability of the message, and MUST return an instance that has the
* new header and/or value.
*
* @param string $name Case-insensitive header field name to add.
* @param string|string[] $value Header value(s).
* @return static
* @throws \InvalidArgumentException for invalid header names or values.
*/
public function withAddedHeader($name, $value)
{
if (is_string($value)) {
$value = [$value];
}
if (!$this->hasHeader($name)) {
return $this->withHeader($name, $value);
}
$this->header->add($name, $value);
return $this;
}
