fix(header): fix fmt_header outputs of several headers

Closes #246

src/header/common/server.rs

@@ -1,5 +1,5 @@
 use header::{Header, HeaderFormat};
-use std::fmt::{self, Show};
+use std::fmt;
 use header::shared::util::from_one_raw_str;
 
 /// The `Server` header field.
@@ -22,8 +22,7 @@ impl Header for Server {
 
 impl HeaderFormat for Server {
     fn fmt_header(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-        let Server(ref value) = *self;
-        value.fmt(fmt)
+        fmt.write_str(&*self.0)
     }
 }