UnitQuaternion::rotation_between gives result with NaN entries for certain nearly-identical inputs

Minimal complete example:

```rust
extern crate nalgebra;

use nalgebra::{UnitQuaternion, Vector3};

fn main() {
// Values derived from real-world occurrence
// Seems to be difficult to reproduce with synthetic values
let axis_0 = Vector3::new(
0.5248905449027862,
-0.30304569551237415,
-0.7953950102334741,
);
let axis_1 = Vector3::new(
// *very* slightly different!
0.5248905432722237,
-0.30304569833659056,
-0.795395010233474,
);

// prints UnitQuaternion with NaN entries:
println!("{:#?}", UnitQuaternion::rotation_between(&axis_0, &axis_1));
}
```