system/HTTP/Message.php
public function setHeader(string $name, $value)
	{
		if (! isset($this->headers[$name]))
		{
			$this->headers[$name] = new Header($name, $value);

			$this->headerMap[strtolower($name)] = $name;

			return $this;
		}

		if (! is_array($this->headers[$name]))
		{
			$this->headers[$name] = [$this->headers[$name]];
		}

		if (isset($this->headers[$name]))
		{
			$this->headers[$name] = new Header($name, $value);
		}

		return $this;
	}
public function setHeader(string $name, $value)
	{
		if (! isset($this->headers[$name]))
		{
			$this->headers[$name] = new Header($name, $value);

			$this->headerMap[strtolower($name)] = $name;

			return $this;
		}

		if (! is_array($this->headers[$name]))
		{
			$this->headers[$name] = [$this->headers[$name]];
		}

		if (isset($this->headers[$name]))
		{
			$this->headers[$name] = new Header($name, $value);
		}

		return $this;
	}
Sets a header and it's value.

@param string            $name
@param array|null|string $value

@return Message|Response