feat(headers): Headers::remove returns the Header

Closes #891

BREAKING CHANGE: `Headers.remove()` used to return a `bool`,
  it now returns `Option<H>`. To determine if a a header exists,
  switch to `Headers.has()`.

src/header/internals/cell.rs

@@ -86,6 +86,20 @@ impl<V: ?Sized + Any + 'static> PtrMapCell<V> {
         }.map(|val| &mut **val)
     }
 
+    #[inline]
+    pub fn into_value(self, key: TypeId) -> Option<Box<V>> {
+        let map = unsafe { self.0.into_inner() };
+        match map {
+            PtrMap::Empty => None,
+            PtrMap::One(id, v) => if id == key {
+                Some(v)
+            } else {
+                None
+            },
+            PtrMap::Many(mut hm) => hm.remove(&key)
+        }
+    }
+
     #[inline]
     pub unsafe fn insert(&self, key: TypeId, val: Box<V>) {
         let mut map = &mut *self.0.get();

src/header/internals/item.rs

@@ -82,6 +82,14 @@ impl Item {
         }
         self.typed.get_mut(tid).map(|typed| unsafe { typed.downcast_mut_unchecked() })
     }
+
+    pub fn into_typed<H: Header>(self) -> Option<H> {
+        let tid = TypeId::of::<H>();
+        match self.typed.into_value(tid) {
+            Some(val) => Some(val),
+            None => parse::<H>(self.raw.as_ref().expect("item.raw must exist")).ok()
+        }.map(|typed| unsafe { typed.downcast_unchecked() })
+    }
 }
 
 #[inline]