Regression in AbstractMapAssert#containsOnlyKeys with Spring's MultiValueMapAdapter #2547

Summary

We saw this when updating from AssertJ 3.19.0 to 3.20.0. It appears that AbstractMapAssert#containsOnlyKeys is mutating the map that we're asserting on, which leads to test failures in our case. This is happening on an instance of org.springframework.util.MultiValueMapAdapter

Example

var underlyingMap = new HashMap<String, List<String>>();
underlyingMap.put("Hello", List.of("World"));

var multiValueMap = CollectionUtils.toMultiValueMap(underlyingMap);
// This assertion passes
assertThat(multiValueMap).containsOnlyKeys("Hello");
// This assertion fails, as `multiValueMap` and `underlyingMap` are now empty
assertThat(multiValueMap).containsOnlyKeys("Hello");

The issue seems to have been introduced in #2167, and is caused by this use of Map#remove on a "clone" of the Map being asserted on. In our case that Map is a Spring MultiValueMapAdapter, which delegates operations to the underlying Map that it was constructed from. The remove call on the clone delegates to multiValueMap#remove which in turn delegates to underlyingMap#remove.