More Rust updates:

- Some stray deriving -> derive changes
- use::{mod} -> use::{self}
- fmt.write -> fmt.write_str

This does not catch the last case of fmt.write_str in the
Show impl of a Header Item. This will need to be changed
separately.

src/header/common/content_length.rs

@@ -1,4 +1,4 @@
-use std::fmt::{mod, Show};
+use std::fmt::{self, Show};
 
 use header::{Header, HeaderFormat};
 use header::shared::util::from_one_raw_str;