Word2007 Writer : Fixed StrikeThrough property

[Progi1984]

Description

Word2007 Writer : Fixed StrikeThrough property

Superseeds #2604 by @noec764

Fixes #1693
Fixes #1722

If a strikethrough style was added to a text element it never appeared in the resulting document.
Indeed, the XML generated was as follows

<w:r>
   <w:rPr>
       <w:strike w:val="1"/>
       <w:dstrike w:val="0"/>
    </w:rPr>
    <w:t xml:space="preserve">some text</w:t>
</w:r>

The statement <w:dstrike w:val="0"/> should not be present here
I figure out that the test $style->isDoubleStrikethrough() !== null is true if $style->isDoubleStrikethrough() === false
So as a text cannot be strike and doublestrike at the same time, word2007 ignores it.

Checklist:

• My CI is 🟢
• I have covered by unit tests my new code (check build/coverage for coverage report)
• I have updated the documentation to describe the changes
• I have updated the changelog

[coveralls]

coverage: 97.04%. remained the same
when pulling 2065d00 on pr2604
into 39e806b on master.

[Progi1984]

Thanks @noec764 for your contribution :)