refactor(XMLCreateOptions): corrected variable names related to null nodes Per https://github.com/oozcitak/xmlbuilder-js/blob/master/CHANGELOG.md#1100---2019-02-18, `skipNullNodes` is replaced with `keepNullNodes` and `skipNullAttributes` with `keepNullAttributes` altering the functionality internally
refactor(XMLCreateOptions): corrected variable names related to null nodes
Per https://github.com/oozcitak/xmlbuilder-js/blob/master/CHANGELOG.md#1100---2019-02-18, `skipNullNodes` is replaced with `keepNullNodes` and `skipNullAttributes` with `keepNullAttributes` altering the functionality internally

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