fix(header): fix fmt_header outputs of several headers

Closes #246

src/header/common/transfer_encoding.rs

@@ -33,7 +33,7 @@ deref!(TransferEncoding => Vec<Encoding>);
 /// # use hyper::header::Headers;
 /// # let mut headers = Headers::new();
 /// headers.set(TransferEncoding(vec![Gzip, Chunked]));
-#[derive(Clone, PartialEq)]
+#[derive(Clone, PartialEq, Show)]
 pub enum Encoding {
     /// The `chunked` encoding.
     Chunked,
@@ -59,12 +59,6 @@ impl fmt::String for Encoding {
     }
 }
 
-impl fmt::Show for Encoding {
-    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-		self.to_string().fmt(fmt)
-	}
-}
-
 impl FromStr for Encoding {
     fn from_str(s: &str) -> Option<Encoding> {
         match s {