Update dataValidation to match spec

Merged snippet requested to merge topic/3.1/issue_1732 into branch/3.1

The spec suggests that showInputMessage and showErrorMessage should be optional and default to False. Updated DataValidation class to match

Ref. https://c-rex.net/projects/samples/ooxml/e1/Part4/OOXML_P4_DOCX_dataValidation_topic_ID0E1DW4.html

Closes #1732 (closed)

This is the same as !418 (closed) but properly rebased to 3.1

--HG-- branch : 3.1

Edited by snippet

Merge request reports