FIX: StrikeThrough property in word2007

[noec764]

I had a problem with strike through in Word2007.
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.

[coveralls]

coverage: 97.217%. remained the same
when pulling bde34d3 on noec764:FIX_StrikeThrough_property_Word2007
into 8b891bb on PHPOffice:master.