Remove the "Don't rewrite dir requests" rule

This rule seems to [trip up users](https://github.com/cakephp/cakephp/issues/4802) more than, I think, it would the other way around.

An invented (though I think this actually comes from past experience) scenario to explain "the problem":

There is a videos controller with an index and a view action. The view action is such that a file is created in the webroot on first access:

**Index listing**:

    url: /videos

Result: Videos index action invoked.

**first view request**:

    url: /videos/view/123.mp4

Request passed to application, video generated and served to the client and stored in the following path:

    filepath: app/webroot/videos/view/123.mp4

**second view request**:

    url: /videos/view/123.mp4

Request directly served by webserver (great, much more efficient)

**Subsequent Index listing**:

    url: /videos

Result:  Only considering apache, an error (server config dependent, either a file listing, a 404 or a 403).

I don't think this is what users actually expect. There is the converse argument that this rule allows users to create:

    filepath: app/webroot/otherapplication/index.html

And access it as 

    url: /otherapplication

But I do not think that is particularly common.

**Should we remove this rule?**

Related: #155