Ported new methods to HttpSessionImpl from SDK repository. (#33)

* Ported new methods to HttpSessionImpl from SDK repository.
diff --git a/lib/src/http_session_impl.dart b/lib/src/http_session_impl.dart
index 8f88a34..1328a96 100644
--- a/lib/src/http_session_impl.dart
+++ b/lib/src/http_session_impl.dart
@@ -74,6 +74,30 @@
     _data.forEach(f);
   }
 
+  void addEntries(Iterable<MapEntry> entries) {
+    _data.addEntries(entries);
+  }
+
+  Iterable<MapEntry> get entries => _data.entries;
+
+  Map<K, V> map<K, V>(MapEntry<K, V> transform(key, value)) =>
+      _data.map(transform);
+
+  void removeWhere(bool test(key, value)) {
+    _data.removeWhere(test);
+  }
+
+  Map<K, V> cast<K, V>() => _data.cast<K, V>();
+
+  Map<K, V> retype<K, V>() => _data.retype<K, V>();
+
+  update(key, update(value), {ifAbsent()}) =>
+      _data.update(key, update, ifAbsent: ifAbsent);
+
+  void updateAll(update(key, value)) {
+    _data.updateAll(update);
+  }
+
   Iterable get keys => _data.keys;
   Iterable get values => _data.values;
   int get length => _data.length;