fix(headers): Remove raw part when getting mutable reference to typed header

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 #821.

src/header/internals/item.rs

@@ -78,6 +78,9 @@ impl Item {
                 Err(_) => ()
             }
         }
+        if self.raw.is_some() && self.typed.get_mut(tid).is_some() {
+            self.raw = OptCell::new(None);
+        }
         self.typed.get_mut(tid).map(|typed| unsafe { typed.downcast_mut_unchecked() })
     }
 }