Create read-only file fails on nightly

cc @alexcrichton 

There has recently been a regression introduced to `std::fs`.  The following line results in `Ok` on stable and `Err` on nightly:

``` rust
std::fs::OpenOptions::new().read(true).create(true).open("some-file")
```

This regression is affecting Linux, Windows, and OS X.  You can see it in action in recent [fs2](https://github.com/danburkert/fs2-rs) CI builds.
