refactor(headers): Improve docs, fix nits, make formatting faster

src/header/parsing.rs now uses unsafe get_unchecked() again, I don't
know why it was removed.

src/header/common/authorization.rs

@@ -1,11 +1,25 @@
 use std::any::Any;
-use std::fmt;
+use std::fmt::{self, Display};
 use std::str::{FromStr, from_utf8};
 use std::ops::{Deref, DerefMut};
 use serialize::base64::{ToBase64, FromBase64, Standard, Config, Newline};
 use header::{Header, HeaderFormat};
 
-/// The `Authorization` header field.
+/// `Authorization` header, defined in [RFC7235](https://tools.ietf.org/html/rfc7235#section-4.2)
+///
+/// The `Authorization` header field allows a user agent to authenticate
+/// itself with an origin server -- usually, but not necessarily, after
+/// receiving a 401 (Unauthorized) response.  Its value consists of
+/// credentials containing the authentication information of the user
+/// agent for the realm of the resource being requested.
+///
+/// # ABNF
+/// ```plain
+/// Authorization = credentials
+/// ```
+///
+/// # Example values
+/// * `Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==`
 #[derive(Clone, PartialEq, Debug)]
 pub struct Authorization<S: Scheme>(pub S);
 
@@ -69,7 +83,7 @@ impl Scheme for String {
     }
 
     fn fmt_scheme(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        fmt::Display::fmt(self, f)
+        Display::fmt(self, f)
     }
 }
 
@@ -97,12 +111,12 @@ impl Scheme for Basic {
         if let Some(ref pass) = self.password {
             text.push_str(&pass[..]);
         }
-        write!(f, "{}", text.as_bytes().to_base64(Config {
+        f.write_str(&text.as_bytes().to_base64(Config {
             char_set: Standard,
             newline: Newline::CRLF,
             pad: true,
             line_length: None
-        }))
+        })[..])
     }
 }