The Request class rework

It seems to me that a possible reason why you do not understand why the get() method works with _POST, and not with _GET, is that you see the http method in the basis of the request, and I see the data.
The get() method works with the current HTTP method. Be it GET/POST/PUST/DELETE/PATCH/etc
That is, get() ! == _GET. get() === get the request data.

get('c', 4) returns 4 because there is no "c" parameter in the current POST request. Therefore, the get() method returns a default value.

I also did not understand your intent.

And I think the `get()` is difficult to understand, because its behavior changes in the context.
I prefer something like `getGet()`, `getPost()`, `getPut()`...

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