GuzzleHttp\Tests\ClientTest::testDoesOverwriteHeaderWithSetRequestOption PHP Method

    public function testDoesOverwriteHeaderWithSetRequestOption()
    {
        $mock = new MockHandler([new Response()]);
        $c = new Client(['headers' => ['User-agent' => 'foo'], 'handler' => $mock]);
        $request = new Request('GET', Server::$url, ['User-Agent' => 'bar']);
        $c->send($request, ['headers' => ['User-Agent' => 'YO']]);
        $this->assertEquals('YO', $mock->getLastRequest()->getHeaderLine('User-Agent'));
    }