Reduce method responsibility

[mapogolions]

When we create an object through the Response constructor

$response = new Response(200)
// protected $this->status = 200
// protected $this->reasonPhrase = '', by default

we don't attempt to guess(set) the value of the reason phrase. We are just using default value(empty string). All needed work is done by the getReasonPhrase method. It maps a status to a reason.

On the other hand, when we call the withStatus method we have a different strategy

$forbidden = new Response(403);
// ...
$ok = $forbidden->withStatus(200);
// protected $this->status = 200;
// protected $this->reasonPhrase = 'OK'

I suggest to rely on the work of the getReasonPhrase method;

[coveralls]

Coverage decreased (-0.0007%) to 99.73% when pulling a46ac36 on mapogolions:enhancement/inconsistent-behaviour into 7936bf9 on slimphp:master.