flask's url_for with _scheme overwrite app's default scheme bug

flask_test.py

# -*- coding: utf-8 -*_
from flask import Flask, url_for


def test():
    # Building test app.
    app = Flask('test')
    app.config['SERVER_NAME'] = 'localhost'

    @app.route('/')
    def index():
        return 'index'

    @app.route('/test')
    def test_view():
        return 'testtesttest'

    with app.test_request_context('/'):
        url = url_for('test_view')
        print(url)  # /test
        url = url_for('test_view', _external=True)
        print(url)  # http://localhost/test
        url = url_for('test_view', _scheme='ws', _external=True)
        print(url)  # ws://localhost/test
        url = url_for('test_view', _external=True)
        print(url)  # bad ws://localhost/test  (expect: http://localhost/test)


test()