#807, appending charset param to Response's 'Content-Type' header

I've added a new setting DEFAULT_CHARSET in rest_framework.settings.DEFAULTS which is None by default (to prevent rest framework from adding the charset param, and stay backwards compatible).

Renderers can have `charset` attribute set on them and it gets appended to the header of the response (if this renderer was used to render it) unless `charset` is set to None.

You can always set `REST_FRAMEWORK['DEFAULT_CHARSET']` in your settings, and it will add the `charset` param to all your responses. You can disable this behavior on a per-renderer basis, by setting individual renderers' `charset` to None.

Added some tests.