[Routing] fix inconsistencies in RequestContext
[Routing] add tests for RequestContext
minor #12190 [Routing] fix type inconsistencies in RequestContext (Tobion)

This PR was merged into the 2.3 branch.

Discussion
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[Routing] fix type inconsistencies in RequestContext

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

Fixed inconsistencies in RequestContext:

- [x] port setters and getters inconsistent with constructor (string vs int)
- [x] host should be lowercased for easier case-insensitivity, see #9072
- [x] fix setQueryString with null
- [x] fix return phpdoc typehint

Commits
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676c4a0 [Routing] add tests for RequestContext
f61607f [Routing] fix inconsistencies in RequestContext