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/cache_control.rs

@@ -3,7 +3,23 @@ use std::str::FromStr;
 use header::{Header, HeaderFormat};
 use header::parsing::{from_one_comma_delimited, fmt_comma_delimited};
 
-/// The Cache-Control header.
+/// `Cache-Control` header, defined in [RFC7234](https://tools.ietf.org/html/rfc7234#section-5.2)
+///
+/// The `Cache-Control` header field is used to specify directives for
+/// caches along the request/response chain.  Such cache directives are
+/// unidirectional in that the presence of a directive in a request does
+/// not imply that the same directive is to be given in the response.
+///
+/// # ABNF
+/// ```plain
+/// Cache-Control   = 1#cache-directive
+/// cache-directive = token [ "=" ( token / quoted-string ) ]
+/// ```
+///
+/// # Example values
+/// * `no-cache`
+/// * `private, community="UCI"`
+/// * `max-age=30`
 #[derive(PartialEq, Clone, Debug)]
 pub struct CacheControl(pub Vec<CacheDirective>);
 
@@ -28,8 +44,8 @@ impl Header for CacheControl {
 }
 
 impl HeaderFormat for CacheControl {
-    fn fmt_header(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-        fmt_comma_delimited(fmt, &self[..])
+    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt_comma_delimited(f, &self[..])
     }
 }