[HttpFoundation] fixed the creation of sub-requests under some circumstances for IIS
merged branch bendavies/iis-rewrite-subrequests (PR #7606)

This PR was merged into the 2.1 branch.

Discussion
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[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.

Commits
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9fcd2f6 [HttpFoundation] fixed the creation of sub-requests under some circumstances for IIS