system/HTTP/Header.php
public function appendValue($value = null)
	{
		if (! is_array($this->value))
		{
			$this->value = [$this->value];
		}

		$this->value[] = $value;

		return $this;
	}
public function appendValue($value = null)
	{
		if (! is_array($this->value))
		{
			$this->value = [$this->value];
		}

		$this->value[] = $value;

		return $this;
	}
Appends a value to the list of values for this header. If the
header is a single value string, it will be converted to an array.

@param null $value

@return $this