fix outdated content_object_init refs

* remove content_object_init section from docs
* improve content_object_init test

The content_object_init signal used to set its class as sender and pass
the instance as additional arg in 6100773. Commit ed907b4 removed this
behaviour to bring it inline with other signals, on the basis that
you can test for the class of the object anyway.
We also had a test in place, checking this behaviour, but it was poorly
implemented, not actually checking if the function ever got called.

closes #1711

pelican/tests/test_contents.py

@@ -1,18 +1,20 @@
-# -*- coding: utf-8 -*-
 from __future__ import unicode_literals, absolute_import
 
-import six
-from sys import platform
 import locale
 import os.path
+import six
 
-from pelican.tests.support import unittest, get_settings
-
-from pelican.contents import Page, Article, Static, URLWrapper, Author, Category
-from pelican.settings import DEFAULT_CONFIG
-from pelican.utils import path_to_url, truncate_html_words, SafeDatetime, posix_join
-from pelican.signals import content_object_init
 from jinja2.utils import generate_lorem_ipsum
+from sys import platform
+
+from pelican.contents import (Page, Article, Static, URLWrapper,
+                              Author, Category)
+from pelican.settings import DEFAULT_CONFIG
+from pelican.signals import content_object_init
+from pelican.tests.support import unittest, get_settings
+from pelican.utils import (path_to_url, truncate_html_words, SafeDatetime,
+                           posix_join)
+
 
 # generate one paragraph, enclosed with <p>
 TEST_CONTENT = str(generate_lorem_ipsum(n=1))
@@ -191,14 +193,19 @@ class TestPage(unittest.TestCase):
         return page_kwargs
 
     def test_signal(self):
-        # If a title is given, it should be used to generate the slug.
-
-        def receiver_test_function(sender, instance):
+        def receiver_test_function(sender):
+            receiver_test_function.has_been_called = True
             pass
+        receiver_test_function.has_been_called = False
 
-        content_object_init.connect(receiver_test_function, sender=Page)
+        content_object_init.connect(receiver_test_function)
+        self.assertIn(
+            receiver_test_function,
+            content_object_init.receivers_for(Page))
+
+        self.assertFalse(receiver_test_function.has_been_called)
         Page(**self.page_kwargs)
-        self.assertTrue(content_object_init.has_receivers_for(Page))
+        self.assertTrue(receiver_test_function.has_been_called)
 
     def test_get_content(self):
         # Test that the content is updated with the relative links to