Fix the PartialEq implementation for Element

The order of attributes in an `Element` doesn't matter anymore.
`<elem a="b" c="d" />` and `<elem c="d" a="b" />` are now correctly
considered equal.

For that I had to derive `PartialOrd` and `Ord` for `Attribute`.
This allows us to sort cloned vectors of `Attribute` in the `PartialEq`
implementation and compare them instead of the struct `attributes`.

Fixes #3

src/element.rs

@@ -20,7 +20,7 @@ use std::slice;
 
 use convert::{IntoElements, IntoAttributeValue, ElementEmitter};
 
-#[derive(Clone, PartialEq, Eq)]
+#[derive(Clone, Eq)]
 /// A struct representing a DOM Element.
 pub struct Element {
     name: String,
@@ -29,6 +29,20 @@ pub struct Element {
     children: Vec<Node>,
 }
 
+impl PartialEq for Element {
+    fn eq(&self, other: &Element) -> bool {
+        let mut my_attr = self.attributes.clone();
+        my_attr.sort();
+        let mut other_attr = other.attributes.clone();
+        other_attr.sort();
+
+        self.name == other.name &&
+        self.namespace == other.namespace &&
+        my_attr == other_attr &&
+        self.children == other.children
+    }
+}
+
 impl fmt::Debug for Element {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         write!(fmt, "<{}", self.name)?;