Resolved issue with rendering of nested serializers form when value is None

Update 2-requests-and-responses.md

just a small error, the trailing slash in the url should be omitted when adding a .suffix

Raise error if passed a serializer instance

Add a method to return the string value used in `choices` of a `RelatedField`

The `display_value` method returns a text type.

Returns the text representation of the instance. Subclasses can override this method to provide a different display value used for populating the `choices` property.

Fixed #3235 - ListField now returns the QueryDict value even if it's a list of only one item.

Fixed #3235 - A querydict which yields a list with only one value now correctly returns the provided data.

Update README with 3.2 release