src/Http/Message.php
public function withoutHeader($name)
    {
        $message = clone $this;

        $name = strtolower((string)$name);
        foreach (array_keys($message->headers) as $k) {
            if ($name == strtolower($k)) {
                unset($message->headers[$k]);
            }
        }

        return $message;
    }
public function withoutHeader($name)
    {
        $message = clone $this;

        $name = strtolower((string)$name);
        foreach (array_keys($message->headers) as $k) {
            if ($name == strtolower($k)) {
                unset($message->headers[$k]);
            }
        }

        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 MessageInterface