async def proxy_handler(request: web.Request) -> web.Response:
    """
    Check request contains http url in query args:
        /fetch?url=http%3A%2F%2Fexample.com%2F
    and trying to fetch it and return body with http status.
    If url passed without scheme or is invalid raise 400 Bad request.
    On failure raise 502 Bad gateway.
    :param request: aiohttp.web.Request to handle
    :return: aiohttp.web.Response
    """
    url = yarl.URL(request.url).query.get('url')
    url_scheme = yarl.URL(url).scheme if url else None
    if not url:
        text, status = 'No url to fetch', 400
    elif not url_scheme:
        text, status = 'Empty url scheme', 400
    elif url_scheme != 'http':
        text, status = 'Bad url scheme: {}'.format(yarl.URL(url).scheme), 400
    else:
        async with request.app['session'] as session:
            response = await session.get(url)
            text, status = await response.text(), response.status
    return web.Response(text=text, status=status)