feat(header): change Cookie to be map-like

The `Cookie` header now has 'set' and `get` methods, to treat the list of
cookies as a map.

Closes #1145

BREAKING CHANGE: The `Cookie` header is no longer a wrapper over a
  `Vec<String>`. It must be accessed via its `get` and `append` methods.

src/header/internals/vec_map.rs

@@ -19,6 +19,11 @@ impl<K: PartialEq, V> VecMap<K, V> {
         }
     }
 
+    #[inline]
+    pub fn append(&mut self, key: K, value: V) {
+        self.vec.push((key, value));
+    }
+
     #[inline]
     pub fn entry(&mut self, key: K) -> Entry<K, V> {
         match self.find(&key) {
@@ -55,10 +60,23 @@ impl<K: PartialEq, V> VecMap<K, V> {
     pub fn iter(&self) -> ::std::slice::Iter<(K, V)> {
         self.vec.iter()
     }
+
     #[inline]
     pub fn remove<K2: PartialEq<K> + ?Sized>(&mut self, key: &K2) -> Option<V> {
         self.find(key).map(|pos| self.vec.remove(pos)).map(|(_, v)| v)
     }
+
+    #[inline]
+    pub fn remove_all<K2: PartialEq<K> + ?Sized>(&mut self, key: &K2) {
+        let len = self.vec.len();
+        for i in (0..len).rev() {
+            if key == &self.vec[i].0 {
+                self.vec.remove(i);
+            }
+        }
+    }
+
+
     #[inline]
     pub fn clear(&mut self) {
         self.vec.clear();