Fix #6385 Request exception

this PR fixes #6385 .

The idea suggested by @weierophinney is to loosen the strictness of `Zend\Http\Request` to get rid of the uncaught exception, and then add an event listener to the mvc cycle to give users some flexibility and feedback (that will be done in a different PR against `develop`).

There is a catch that I leave for discussion, tho.

Personally, I can't see any reason why one would want to have `Application::construct()` trigger a fatal error if a bogus http method was set in the server environment, but - although unlikely - it's possible that people could rely on the exceptions thrown by `Request::setMethod()` and `Request::fromString()` when using `Zend\Http` component alone. One example is `Zend\Http\Client` itself, as shown in this PR.

It may be worth mentioning that this behaviour wasn't covered by the `Request` tests themselves, but it was in `Client` ones. Overall I'd say dropping some strictness seems legit, but I can see why it may be undesirable.

As an alternative, it's totally possible to set the flag strict by default, and open it up in `Zend\Mvc\Service\RequestFactory` instead. That could preserve the old behaviour of `Zend\Http` component and at the same time make `Application` behaviour more sane.