Flask is unreachable: _get_framework only accepts bottle, mk_flask_app ignores it #39

Found in the review of PR #38 (#37). _get_framework raises NotImplementedError for any framework but bottle and forces PY2HTTP_FRAMEWORK=bottle, so mk_app(..., framework='flask') raises, and mk_flask_app([f]) called directly imports bottle's request and routes errors to the bottle handler (Flask answers 200 with an error body). flask_error_handler is now correct but unreachable from any py2http builder. Needed: accept flask in _get_framework (without overwriting the env var for other frameworks), make the flask routes use flask's request/handler, and add an end-to-end mk_app(framework='flask') test for statuses and WWW-Authenticate. Check the dependents before widening the framework contract.