This commit allows `response.render` to be called when
`response.rendered_context == {}`. This should be allowed, since
if [the JSONRenderer, for example](https://github.com/tomchristie/django-rest-framework/blob/master/rest_framework/renderers.py#L85-L92) receives a `None` context, it
sets it to an empty dictionary itself.

In commit 5392be4d, there was a change
made when cleaning up the template for the API listing that caused 2
spaces to appear before every header item (except the first) and before
the first line of the body of the response. This meant that it often
looked like:

HTTP 200 OK
  Allow: GET, OPTIONS
  Content-Type: application/json
  Vary: Accept

  {
    "key": "value",
    "key2": "value2"
}

This change removes those leading spaces, so that it will now look like:

HTTP 200 OK
Allow: GET, OPTIONS
Content-Type: application/json
Vary: Accept

{
    "key": "value",
    "key2": "value2"
}

* Don't deepcopy 'regex' arguments, instead treat as immutable.

fix a b0rked code example in the permissions section of api guide

It seems that the example was copy&pasted and not updated properly.

Pass request to schema generation

Stricter type validation for CharField

Update RelatedField.choices to support non-string values