Document other Collection gotchas

[ljacqu]

The tests focus on differences among implementations of the same interface, but it would be interesting to highlight some of the less obvious behavior that all implementations [should] share. Such as:

• List#remove removes only the first element that matches, List#removeAll removes them all
• Map#putIfAbsent behavior if the Function returns null
• Wasn't there some oddity that #toArray doesn't always return Object[]? Maybe only in one impl?
• toArray sets the next elem to null if available

• equals among collection types deserves its own test (probably its own issue)

[ljacqu]

reversed doesn't behave the same for ArrayList vs LinkedHashSet

    public static void main(String[] args) {
        {
            LinkedHashSet<String> seqColl = new LinkedHashSet<>(Arrays.asList("b"));
            SequencedCollection<String> reversedCollection = seqColl.reversed();
            reversedCollection.add("f");
            System.out.println(reversedCollection); // Output: [f, b]
        }

        {
            ArrayList<String> seqColl = new ArrayList<>(Arrays.asList("b"));
            SequencedCollection<String> reversedCollection = seqColl.reversed();
            reversedCollection.add("f");
            System.out.println(reversedCollection); // Output: [b, f]
        }
    }