feat(headers): Implement Content-Language header field

Closes #475

src/header/shared/language.rs

@@ -0,0 +1,45 @@
+use std::str::FromStr;
+use std::fmt;
+
+/// A language tag.
+/// See http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.10
+///
+/// Note: This is no complete language tag implementation, it should be replaced with
+/// github.com/pyfisch/rust-language-tag once it is ready.
+#[derive(Clone, PartialEq, Debug)]
+pub struct Language {
+    /// The language tag
+    pub primary: String,
+    /// A language subtag or country code
+    pub sub: Option<String>
+}
+
+impl FromStr for Language {
+    type Err = ();
+    fn from_str(s: &str) -> Result<Language, ()> {
+        let mut i = s.split("-");
+        let p = i.next();
+        let s = i.next();
+        match (p, s) {
+            (Some(p), Some(s)) => Ok(Language {
+                primary: p.to_string(),
+                sub: Some(s.to_string())
+                }),
+            (Some(p), _) => Ok(Language {
+                primary: p.to_string(),
+                sub: None
+                }),
+            _ => Err(())
+        }
+    }
+}
+
+impl fmt::Display for Language {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        try!(write!(f, "{}", self.primary));
+        match self.sub {
+            Some(ref s) => write!(f, "-{}", s),
+            None => Ok(())
+        }
+    }
+}