fix get_by_name call in flask_server.py.

[GitHubNull]

The call get_by_name in

captcha_platform/flask_server.py

Line 172 in a6a1ae0

interface = interface_manager.get_by_name(size_string, request.json['model_name'])

is wrong. If use it by the way like that will never can not get other model interface, and only can get the default model interface, if default model interface is define.