Default stickynav to on properly.

This is documented as being on,
but previously we weren't running `tobool` on it to check it properly.
This had the effect of always returning True unless the user set it to False in Python.

You can see the commit that changed it here: https://github.com/rtfd/sphinx_rtd_theme/pull/496/files#diff-1336f80431f476a232e3c48abb277c51R210

This properly defaults it to True,
so that navigation works :)

README.rst

@@ -87,17 +87,17 @@ file of this repository, and can be defined in your project's ``conf.py`` via
 .. code:: python
 
     html_theme_options = {
-        'typekit_id': hiw1hhg,
-        'canonical_url':
-        'analytics_id':
-        'collapse_navigation': False
-        'sticky_navigation': False
-        'navigation_depth': 4
-        'includehidden': True
-        'logo_only':
-        'display_version': True
+        'typekit_id': '',
+        'canonical_url': '',
+        'analytics_id': '',
+        'collapse_navigation': False,
+        'sticky_navigation': True,
+        'navigation_depth': 4,
+        'includehidden': True,
+        'logo_only': False,
+        'display_version': True,
         'prev_next_buttons_location': bottom,
-        'style_external_links': False
+        'style_external_links': False,
     }
 
 The following options are available:

sphinx_rtd_theme/theme.conf

@@ -8,7 +8,7 @@ typekit_id = hiw1hhg
 canonical_url =
 analytics_id =
 collapse_navigation = False
-sticky_navigation = False
+sticky_navigation = True
 navigation_depth = 4
 includehidden = True
 logo_only =