fix(headers): kill raw part when getting mutable reference to typed h…

…eader

If you get a mutable reference to a typed header it is possible to make
the two representations be out of sync. To avoid this, after parsing the
raw part it should be removed.

Fixes hyperium#821.

src/header/internals/item.rs

@@ -73,6 +73,7 @@ impl Item {
         if self.typed.get_mut(tid).is_none() {
             match parse::<H>(self.raw.as_ref().expect("item.raw must exist")) {
                 Ok(typed) => {
+                    self.raw = OptCell::new(None);
                     unsafe { self.typed.insert(tid, typed); }
                 },
                 Err(_) => ()