[HttpFoundation] getClientIp is fixed.

The getClientIp now returns ip of the earliest server in a proxy chain when all the servers in the chain are trusted proxies. Before this patch the getClientIp used to return null at such condition.
Some appropriate tests are added.
merged branch denvned/getClientIp-fix (PR #7472)

This PR was merged into the 2.1 branch.

Discussion
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[HttpFoundation] getClientIp is fixed.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | no

The getClientIp now returns ip of the earliest server in a proxy chain when all the servers in the chain are trusted proxies. Before this patch the getClientIp used to return null at such condition.
Some appropriate tests are added.

Commits
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c4da2d9 [HttpFoundation] getClientIp is fixed.