store creds correctly when authorized_user_filename is passed to gspread.auth.oauth()

[sarisia]

Summary

#847 allows users to use custom path for authorized user credentials.

With current implementation, when I call gspread.auth.oauth() with custom path,
oauth() reads credentials from the given path correctly, but if credentials file is missing,
it authorizes user and store the credentials to the default path (~/.config/gspread/credentials.json),
not the custom path!

According to the docs, this behavior is incorrect:
https://github.com/burnash/gspread/blob/master/docs/oauth2.rst

If you want to store the credentials file somewhere else, specify the path to authorized_user.json

This PR fixes store_credentials() in gspread.auth.oauth() to store creds correctly when authorized_user_filename is passed to gspread.auth.oauth().

[lavigne958]

Test ✔️

[lavigne958]

Thank you @sarisia for this contribution 🙂