MYFACESTEST-17 Update MockExternalContext so it delegate to mock request and response objects and add new methods present on facelets mocks (minor fix: changed MockSessionMap to _SessionMap on MockExternalContext, because MockSessionMap is broken on some operations)

Furthermore I think this was forgotten to change, because MockRequestMap was changed to _RequestMap and MockApplicationMap was changed to _ApplicationMap, but MockSessionMap was not changed.
diff --git a/test12/src/main/java/org/apache/myfaces/test/mock/MockExternalContext.java b/test12/src/main/java/org/apache/myfaces/test/mock/MockExternalContext.java
index 9b61d28..b2592b2 100644
--- a/test12/src/main/java/org/apache/myfaces/test/mock/MockExternalContext.java
+++ b/test12/src/main/java/org/apache/myfaces/test/mock/MockExternalContext.java
@@ -430,8 +430,7 @@
     {
         if (sessionMap == null)
         {
-            HttpSession session = request.getSession(true);
-            sessionMap = new MockSessionMap(session);
+            sessionMap = new _SessionMap(request);
         }
         return sessionMap;
     }