Flask: non-JSON input mappers, send_json_resp mappers and empty bodies #40

Follow-ups from the review of the flask support (#39, PR for it): under framework='flask' (1) handle_raw_req, handle_binary_req and the form mapper read Bottle's request (request.body, request.files) and fail per request with a 500 whose text leaks the AttributeError; they should fail at build time or be ported; (2) a user output mapper made with @send_json_resp sets Bottle's response.content_type, so Flask answers text/html; (3) an empty or non-JSON body gives a 500 with werkzeug's text instead of a 400 InputError; (4) Bottle-only configs (plugins, enable_cors, publish_swagger, openapi_insecure) are refused for flask; supporting them (JWT plugin equivalent as a before_request hook, CORS) is the real work.