Fixes https://github.com/spring-projects/spring-integration/issues/3641
When a duplicate cookie name appears in a request, an `IllegalStateException` is thrown.
The default `Collectors.toMap()` does not allow a duplicated keys.
* Handle `servletRequest.getCookies()` as a `MultiValueMap`
* Call `toSingleValueMap()` for the evaluation context variable to restore previous behavior.
The next major version must expose the `MultiValueMap` as is to give access to all cookies from end-user expressions
* Rework some HTTP tests to JUnit 5
**Cherry-pick to `5.4.x` & `5.3.x`**
7e5fef9e45