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/access_control_allow_methods.rs

@@ -2,7 +2,7 @@ use method::Method;
 
 header! {
     #[doc="`Access-Control-Allow-Methods` header, part of"]
-    #[doc="[CORS](www.w3.org/TR/cors/#access-control-allow-methods-response-header)"]
+    #[doc="[CORS](http://www.w3.org/TR/cors/#access-control-allow-methods-response-header)"]
     #[doc=""]
     #[doc="The `Access-Control-Allow-Methods` header indicates, as part of the"]
     #[doc="response to a preflight request, which methods can be used during the"]