[Routing] fix inconsistencies in RequestContext

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | partly
| 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] <del>some methods were tagged with `@api` which makes no sense since the class is already tagged as stable. this automatically includes all methods. also our bc promise is only about `@api` on classes</del>
- [x] setParameters the only fluent method! I made them all fluent which makes sense for this class

IMO this should be merged into 2.3 to reduce the divergence.
Shouldn't this be ``self``?