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/shared/encoding.rs

@@ -24,8 +24,8 @@ pub enum Encoding {
 }
 
 impl fmt::Display for Encoding {
-    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-        fmt.write_str(match *self {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str(match *self {
             Chunked => "chunked",
             Gzip => "gzip",
             Deflate => "deflate",