fixes invalid format of mime types

See also mime.rs/#14

src/header/common/content_type.rs

@@ -1,5 +1,5 @@
 use header::{Header, HeaderFormat};
-use std::fmt::{self, Show};
+use std::fmt::{self, String};
 use header::shared::util::from_one_raw_str;
 use mime::Mime;