src/Message/Message.php
public function withoutHeader($name)
    {
        $message = clone $this;
        $key = $this->headerKey($name);

        if (array_key_exists($key, $message->headers)) {
            unset($message->headers[$key]);
        }

        return $message;
    }
public function withoutHeader($name)
    {
        $message = clone $this;
        $key = $this->headerKey($name);

        if (array_key_exists($key, $message->headers)) {
            unset($message->headers[$key]);
        }

        return $message;
    }
Return an instance without the specified header.

Header resolution MUST be done without case-sensitivity.

This method MUST be implemented in such a way as to retain the
immutability of the message, and MUST return an instance that removes
the named header.

@param string $name Case-insensitive header field name to remove.
@return static