[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