feat(headers): add AcceptCharset header

Adds support for the Accept-Charset header.  Encodes the charset as a
string.

src/header/common/accept_charset.rs

@@ -0,0 +1,27 @@
+use header::{self, QualityItem};
+
+pub type Charset = String;
+
+/// The `Accept-Charset` header
+///
+/// The `Accept-Charset` header can be used by clients to indicate what
+/// response charsets they accept.
+#[derive(Clone, PartialEq, Debug)]
+pub struct AcceptCharset(pub Vec<QualityItem<Charset>>);
+
+impl_list_header!(AcceptCharset,
+                  "Accept-Charset",
+                  Vec<QualityItem<Charset>>);
+
+
+#[test]
+fn test_parse_header() {
+    let a: AcceptCharset = header::Header::parse_header(
+        [b"iso-8859-5, unicode-1-1;q=0.8".to_vec()].as_slice()).unwrap();
+    let b = AcceptCharset(vec![
+        QualityItem{item: "iso-8859-5".to_string(), quality: 1.0},
+        QualityItem{item: "unicode-1-1".to_string(), quality: 0.8},
+    ]);
+    assert_eq!(format!("{}", a), format!("{}", b));
+    assert_eq!(a, b);
+}