rustdoc: unmarked code fences are doctests too

#20075 introduced a bug where unmarked code fences weren't considered as doctests. This PR fixes the logic.

---

This passed `check-stage1-rustdoc`, and I manually checked that:

``` rust
//! ```
//! println!("Hello")
//! ```
//!
//! ``` rust
//! println!("Hello")
//! ```
//!
//! ``` sh
//! println!("Hello")
//! ```
//!
//! ``` ignore
//! println!("Hello")
//! ```
```

Generated:

``` rust
running 3 tests
test _2 ... ignored
test _0 ... ok
test _1 ... ok
```

I'd love to add that as a test, but I have no idea how to do that with our testing infrastructure. If anyone knows how, do let me know!

r? @alexcrichton 
@seanmonstar feeback?