Add spec to ensure that Array#eql? checks type of argument

This spec was prompted by a difference I discovered between `Array#eql?` in MRI and JRuby.

MRI's `Array#eql?` checks the type of the argument and returns `false` if it is not an `Array` or subclass. JRuby checks if the argument responds to `#to_ary`, and if so, flips the receiver and argument around and calls `#==`.