SPR-15140: @RequestParam injects a raw undecoded string for reactive HTTP requests

[eiiches]

When using spring-web-reactive, %-encoded strings were injected as-is into @RequestParam variables, which does not coincide with spring-webmvc behaviour.

Example:

@ResponseBody
@GetMapping("/search")
public String search(@RequestParam("q") String q) {
    return q;
}

curl -s 'http://localhost:8080/search?q=%20%2B+%C3%A0'

Spring Web Reactive: %20%2B+%C3%A0
Spring Web MVC: (space)+ à

((space) is a space character, GitHub seems to remove leading whitespaces...)

---

The cause is that AbstractServerHttpRequest.getQueryParams() is returning undecoded name-value pairs, while the interface method ServerHttpRequest.getQueryParams() specifies (in javadoc) to return decoded ones.

This commit fixed the AbstractServerHttpRequest implementation to correctly return
decoded name-value pairs and added an unit test.

https://jira.spring.io/browse/SPR-15140

(I have signed the CLA.)

[rstoyanchev]

This is merged, thanks for the report and pull request.

Note that I've added an integration test as well and fixed the resulting issues. Also I replaced the use of URLDecoder which is not really for urls despite its name (see Javadoc) and replaced it with UriUtils which is what we commonly use.