Fix emdey free courses download.

[awei78]

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It seems that Udemy has recently been revamped. When downloaded with youtube-dl, it prompts:

[udemy:course] the-ultimate-microsoft-office-2016-training-bundle: Downloading w
ebpage
ERROR: Unable to download webpage: HTTP Error 403: Unauthorized (caused by HTTPE
rror()); please report this issue on https://yt-dl.org/bug . Make sure you are u
sing the latest version; type  youtube-dl -U  to update. Be sure to call youtube
-dl with the --verbose flag and include its complete output.

I searched all issues but no solution, only the duplicate flag marked by @dstftw.
How to solve it? modify the file udemy.py:
line 124, the _download_webpage_handle funcion, add Authorization in headers:

    def _download_webpage_handle(self, *args, **kwargs):
        headers = kwargs.get('headers', {}).copy()
        headers['User-Agent'] = 'Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.21 (KHTML, like Gecko) Mwendo/1.1.5 Safari/537.21'
        headers['X-Requested-With'] = 'XMLHttpRequest'
        headers['Host'] = 'www.udemy.com'
        for cookie in self._downloader.cookiejar:
            if cookie.name == 'access_token':
                headers['Authorization'] = cookie.value
                break
        else:
        # This header is taken from https://github.com/FaisalUmair/udemy-downloader-gui thanks to @FaisalUmair for quick help. thank umdey-dl
            headers['Authorization'] = 'Basic YWQxMmVjYTljYmUxN2FmYWM2MjU5ZmU1ZDk4NDcxYTY6YTdjNjMwNjQ2MzA4ODI0YjIzMDFmZGI2MGVjZmQ4YTA5NDdlODJkNQ=='
        kwargs['headers'] = headers
        return super(UdemyIE, self)._download_webpage_handle(
            *args, **compat_kwargs(kwargs))

OK, enjoy it! 

Note: Before using it, you need an account to log in to udemy. and, can only download free courses.