make sure 'base_manager is not inherited (but managed by Django instead).

(This seems more correct but it doen't seem to make any difference.)
Also added related test cases.

README.html

@@ -213,7 +213,6 @@ ul.auto-toc {
 
 <div class="section" id="polymorphic-models-for-django">
 <h1>Polymorphic Models for Django</h1>
-<p>.</p>
 <div class="section" id="quick-start-docs-contributing">
 <h2>Quick Start, Docs, Contributing</h2>
 <ul class="simple">