[HttpFoundation] Added Request::getEncodings() method
merged branch msonnabaum/request-get-encodings (PR #8185)

This PR was merged into the master branch.

Discussion
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[HttpFoundation] Added Request::getEncodings() method

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

We needed access to the Accept-Encoding information in Drupal, and I was surprised to see that there wasn't a method on the request object to access this.

This PR adds that method.

Commits
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28a8443 [HttpFoundation] Added Request::getEncodings() method
[HttpFoundation] tweaked previous merge (refs #8185)