Add class_exists() checks to CI_Loader::model()

Helps debugging in case of controller/model/library class name collision.
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index b6ad148..dcbe820 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -34,6 +34,7 @@
 -  Core
 
    -  Added support for defining a list of specific query parameters in ``$config['cache_query_string']`` for the :doc:`Output Library <libraries/output>`.
+   -  Added class existence and inheritance checks to ``CI_Loader::model()`` in order to ease debugging in case of name collisions.
 
 Bug fixes for 3.0.1
 -------------------