Make use of late static binding to allow decorator extension

[trvrnrth]

We would like to extend the response decorator in order to add additional domain-specific helpers. Unfortunately this is not possible at present as the current helpers do not make use of late static binding.

Using the response as an example, the current helpers take the basic format of:

/**
 * @return Response
 */
public function withX(): ResponseInterface
{
    $response = $this->response->withY();
    return new Response($response, $this->streamFactory);
}

In order to allow extension it would be nice if these made use of late static binding to type hint and create the new decorated response as follows:

/**
 * @return static
 */
public function withX(): ResponseInterface
{
    $response = $this->response->withY();
    return new static($response, $this->streamFactory);
}

[coveralls]

Coverage remained the same at 99.275% when pulling 7586b67 on blubolt:late-static-binding into ff98230 on slimphp:master.

[l0gicgate]

Looks good to me @akrabat if you want to merge

[l0gicgate]

@trvrnrth please fix conflicts that arose from merging your other PRs and I’ll merge.

[trvrnrth]

@l0gicgate Resolved.

Thanks all for taking the time to get these in.