Tests/struct question mark

While developing some software on 1.8, I discovered that question marks in struct members break everything.

I assumed it was still broken, so I wrote a test and found it's fixed on trunk, but since there's no regression test for it I'm pushing it upstream.

The error on 1.8:

```
irb(main):001:0> k = Struct.new(:a, :b?)
=> #<Class:0x107387108>
irb(main):002:0> o = k.new("a", false)
=> #<struct #<Class:0x107387108> a="a", :b?=false>
irb(main):003:0> o.b?
NoMethodError: undefined method `b?' for #<struct #<Class:0x107387108> a="a", :b?=false>
        from (irb):3
irb(main):004:0>
```