Replacement memory leak hotfix

[GrahamCampbell]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets	fixes #184
License	MIT

Original description:

There is a big memory leak when using the PluginClient with plugins.

Example code to reproduce the issue:

use Http\Client\Curl\Client;

$samples = 50;
$curlClient = new Client();
$requestFactory = $container->get(\Psr\Http\Message\RequestFactoryInterface::class);
$pluginClient = new \Http\Client\Common\PluginClient(
    $curlClient,
    [
        new \Http\Client\Common\Plugin\RedirectPlugin(),
    ]
);

$request = $requestFactory->createRequest('GET', 'https://google.it');
for ($i = 1; $i <= $samples; $i++) {
    $pluginClient->sendRequest($request);
    echo 'memory: ' . memory_get_usage() . PHP_EOL;
    // gc_collect_cycles(); without this, gc isn't able to free memory
}

This PR fixes it removing useless variable reference.

[dbu]

can you please add a note to the changelog? this could also help in case there is some unexpected side effect, to help people figure out what to look at if they see a problem after upgrading.

[GrahamCampbell]

I'll send a change log PR to add everything at the end if that's ok? This is not the only change worth noting since the last release. :)

[dbu]

i usually try to keep the changelog updated constantly, as otherwise these things get forgotten. but with all the parallel things you do that would mean conflicts and rebases after every merge, and i trust you to not forget it at the end :-)