file_generator for l120 of rest.py

rest.py.edit

            elif isinstance(val, cherrypy.lib.file_generator):
                # if endpoint returned a file generator, return it directly
                #
                #  Example
                #
                # > from cherrypy.lib import file_generator
                # > cherrypy.response.headers['Content-Type'] = "image/png"
                # > png_handle = open(file_path, 'rb')
                # > return file_generator(png_handle)
                #
                #
                return val