make sure, equivalentValues(Object) is used if available

[rbri]

The HTMLAllCollection implements some strange behavior - https://developer.mozilla.org/en-US/docs/Web/API/HTMLAllCollection#special_type_conversion_behavior. Of course we have to support this in HtmlUnit - see HtmlUnit/htmlunit#896

Luckily Rhino supports this kind of customized equals behavior by overriding the ScriptableObject#equivalentValues() method.

But there are two bugs

• the equivalentValues() method is called if the lhs is null or undefined but NOT if the rhs is null or undefined (interpreter).
• the optimized version works correct if the lhs is undefined or rhs is undefined but does not call the equivalentValues() method in case lhs/rhs is null

This PR fixes both cases and also adds test cases.

[rbri]

any feedback for this? I need it for HtmlUnit and i like to know if there is a chance to have it in rhino or if i have to put it in my fork.

I did already some test and i did not see any negative performance impact.

[gbrail]

This makes sense to me -- I've been travelling a bit and haven't had a chance to test things myself yet. I'm also optimistic that a few of the optimizations like the ones in the bytecode here that you had to undo might be less necessary as we can keep working on dynamic linking -- I'll have some PRs for that coming up in the future. For now I'll check this out in detail later in the week.