[HttpFoundation] fixes creation of sub requests under IIS & Rewite Module

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6936, #6923
| License       | MIT
| Doc PR        | N/A

There are a few bugs to address.

1. HTTP_X_ORIGINAL_URL wasn't removed from the server parameters, so is picked back up [here](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpFoundation/ServerBag.php#L33) upon recreation of a sub request.
2. When `X_ORIGINAL_URL` is passed in the headers by IIS, `IIS_WasUrlRewritten` and `UNENCODED_URL` can also be passed as server vars, so they must also be removed for sub request URI's to be resolved correctly.

Additionally, I have removed the OS check for windows, because it was only done for 2 out of 4 of the IIS specific checks, and it made the code untestable.

Also added tests for all scenarios as there were none.