[1.9] Array#join using to_a instead of to_s when both defined

When both the #to_a and #to_s methods are defined on an object in an array which #join is called on, #to_a is used instead of #to_s for determining the string value of the object which is joined. The return value of the #to_a call also seems to be flattened with the containing array before the join is applied.

This is causing a problem with url slug generation of Mongoid documents in rails 3.

A test/unit testcase for this issue is attached. This issue only occurs when enabling 1.9 support with --1.9. This test passes on MRI 1.9.2 and JRuby 1.6RC1 --1.8.