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/set_cookie.rs

@@ -1,15 +1,59 @@
 use header::{Header, HeaderFormat};
-use std::fmt;
+use std::fmt::{self, Display};
 use std::str::from_utf8;
 
 use cookie::Cookie;
 use cookie::CookieJar;
 
-/// The `Set-Cookie` header
+/// `Set-Cookie` header, defined [RFC6265](http://tools.ietf.org/html/rfc6265#section-4.1)
+///
+/// The Set-Cookie HTTP response header is used to send cookies from the
+/// server to the user agent.
 ///
 /// Informally, the Set-Cookie response header contains the header name
 /// "Set-Cookie" followed by a ":" and a cookie.  Each cookie begins with
 /// a name-value-pair, followed by zero or more attribute-value pairs.
+///
+/// # ABNF
+/// ```plain
+///  set-cookie-header = "Set-Cookie:" SP set-cookie-string
+/// set-cookie-string = cookie-pair *( ";" SP cookie-av )
+/// cookie-pair       = cookie-name "=" cookie-value
+/// cookie-name       = token
+/// cookie-value      = *cookie-octet / ( DQUOTE *cookie-octet DQUOTE )
+/// cookie-octet      = %x21 / %x23-2B / %x2D-3A / %x3C-5B / %x5D-7E
+///                       ; US-ASCII characters excluding CTLs,
+///                       ; whitespace DQUOTE, comma, semicolon,
+///                       ; and backslash
+/// token             = <token, defined in [RFC2616], Section 2.2>
+///
+/// cookie-av         = expires-av / max-age-av / domain-av /
+///                    path-av / secure-av / httponly-av /
+///                     extension-av
+/// expires-av        = "Expires=" sane-cookie-date
+/// sane-cookie-date  = <rfc1123-date, defined in [RFC2616], Section 3.3.1>
+/// max-age-av        = "Max-Age=" non-zero-digit *DIGIT
+///                       ; In practice, both expires-av and max-age-av
+///                       ; are limited to dates representable by the
+///                       ; user agent.
+/// non-zero-digit    = %x31-39
+///                       ; digits 1 through 9
+/// domain-av         = "Domain=" domain-value
+/// domain-value      = <subdomain>
+///                       ; defined in [RFC1034], Section 3.5, as
+///                       ; enhanced by [RFC1123], Section 2.1
+/// path-av           = "Path=" path-value
+/// path-value        = <any CHAR except CTLs or ";">
+/// secure-av         = "Secure"
+/// httponly-av       = "HttpOnly"
+/// extension-av      = <any CHAR except CTLs or ";">
+/// ```
+///
+/// # Example values
+/// * `SID=31d4d96e407aad42`
+/// * `lang=en-US; Expires=Wed, 09 Jun 2021 10:18:14 GMT`
+/// * `lang=; Expires=Sun, 06 Nov 1994 08:49:37 GMT`
+/// * `lang=en-US; Path=/; Domain=example.com`
 #[derive(Clone, PartialEq, Debug)]
 pub struct SetCookie(pub Vec<Cookie>);
 
@@ -46,7 +90,7 @@ impl HeaderFormat for SetCookie {
             if i != 0 {
                 try!(f.write_str("\r\nSet-Cookie: "));
             }
-            try!(write!(f, "{}", cookie));
+            try!(Display::fmt(cookie, f));
         }
         Ok(())
     }