Changed action="." to action="" in tests and docs.

`action="."` strips query parameters from the URL which is not usually what
you want. Copy-paste coding of these examples could lead to difficult to
track down bugs or even data loss if the query parameter was meant to alter
the scope of a form's POST request.

docs/ref/csrf.txt

@@ -40,7 +40,7 @@ To take advantage of CSRF protection in your views, follow these steps:
 2. In any template that uses a POST form, use the :ttag:`csrf_token` tag inside
    the ``<form>`` element if the form is for an internal URL, e.g.::
 
-       <form action="." method="post">{% csrf_token %}
+       <form action="" method="post">{% csrf_token %}
 
    This should not be done for POST forms that target external URLs, since
    that would cause the CSRF token to be leaked, leading to a vulnerability.