feat(headers): Implement PartialOrd for QualityItem

PartialOrd is useful to sort Collections of QualityItems by quality.
So you can find the preferred match easier.

Closes #314

src/header/shared/quality_item.rs

@@ -5,6 +5,7 @@
 
 use std::fmt;
 use std::str;
+use std::cmp;
 #[cfg(test)] use super::encoding::*;
 
 /// Represents an item with a quality value as defined in
@@ -26,6 +27,12 @@ impl<T> QualityItem<T> {
     }
 }
 
+impl<T: PartialEq> cmp::PartialOrd for QualityItem<T> {
+    fn partial_cmp(&self, other: &QualityItem<T>) -> Option<cmp::Ordering> {
+        self.quality.partial_cmp(&other.quality)
+    }
+}
+
 impl<T: fmt::Display> fmt::Display for QualityItem<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         if self.quality == 1.0 {
@@ -127,3 +134,10 @@ fn test_quality_item_from_str5() {
     let x: Result<QualityItem<Encoding>, ()> = "gzip; q=0.2739999".parse();
     assert_eq!(x, Err(()));
 }
+#[test]
+fn test_quality_item_ordering() {
+    let x: QualityItem<Encoding> = "gzip; q=0.5".parse().ok().unwrap();
+    let y: QualityItem<Encoding> = "gzip; q=0.273".parse().ok().unwrap();
+    let comparision_result: bool = x.gt(&y);
+    assert_eq!(comparision_result, true)
+}