OrderedFloat PartialOrd implementation should match Ord implementation

[cake4289]

Otherwise OrderedFloat is not ordered at all, breaking the assumptions of any algorithm that uses it.

The documentation for Ord says

Implementations of PartialEq, PartialOrd, and Ord must agree with each other. It's easy to accidentally make them disagree by deriving some of the traits and manually implementing others.

The following test fails:

extern crate ordered_float;

use std::f64::NAN;
use ordered_float::OrderedFloat;

#[test]
fn less_than_nan() {
    let x = OrderedFloat(1.0);
    let y = OrderedFloat(NAN);
    assert!(x < y);
}

None of the tests in this crate cover using the comparison operators; they all use Ord::cmp directly. This should be fixed. The current code is unsound and could lead to unsafety.

[mbrubeck]

This is fixed on master.