Improve HTTP test coverage

This pull request increases test coverage in `HttpRequestTest` and `HttpResponseTest` for `Illuminate\HTTP\Request`, `Illuminate\HTTP\Response` and `Illuminate\HTTP\RedirectResponse`.

Before:

```
\Illuminate\Http::RedirectResponse
  Methods:  58.33% ( 7/12)   Lines:  61.29% ( 19/ 31)
\Illuminate\Http::Request
  Methods:  69.23% (27/39)   Lines:  71.43% ( 65/ 91)
\Illuminate\Http::Response
  Methods:  50.00% ( 3/ 6)   Lines:  70.00% ( 14/ 20)
```

After:

```
\Illuminate\Http::RedirectResponse
  Methods: 100.00% (12/12)   Lines: 100.00% ( 31/ 31)
\Illuminate\Http::Request
  Methods:  89.74% (35/39)   Lines:  86.81% ( 79/ 91)
\Illuminate\Http::Response
  Methods: 100.00% ( 6/ 6)   Lines: 100.00% ( 20/ 20)
```

Branching properties in `Illuminate\Http\Request::createFromBase()` made it hard to get at, and the flash methods in `Illuminate\Http\Request` were also neglected, but otherwise, all methods previously marked by PHPUnit as uncovered for these three classes should now be covered.

Additionally, unused method argument for `Illuminate\Http\Request->is()` was removed and the comment updated to reference its actual usage.