Ignore whitespace in sharedStrings.xml

[afalkenhahn]

I have some XLSX files in which sharedStrings.xml is formatted like this:

<?xml version="1.0"?><sst xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" count="1" uniqueCount="1">
  <si>
    <t/>
  </si>
</sst>

The whitespace in the XML will make OpenXLSX misbehave. In fact, this XML will cause OpenXLSX to add three items to m_sharedStringCache even though there is only string in the XML. Adding three items will totally screw up the string index mappings so this is quite a critical issue which should be fixed.