Added CI_Output::get_header()

(an improved version of PR #645)

Also fixed get_content_type() to only return the MIME value and created
Output library unit tests for both of these methods.
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index ab1d99d..09c425d 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -294,8 +294,9 @@
    -  Added support for HTTP-Only cookies with new config option *cookie_httponly* (default FALSE).
    -  Renamed method ``_call_hook()`` to ``call_hook()`` in the :doc:`Hooks Library <general/hooks>`.
    -  :doc:`Output Library <libraries/output>` changes include:
-	 -  Added method ``get_content_type()``.
 	 -  Added a second argument to method ``set_content_type()`` that allows setting the document charset as well.
+	 -  Added method ``get_content_type()``.
+	 -  Added method ``get_header()``.
    -  ``$config['time_reference']`` now supports all timezone strings supported by PHP.
    -  :doc:`Config Library <libraries/config>` changes include:
 	 -  Changed ``site_url()`` method  to accept an array as well.