NcxResource does not account for namespaces

[adduc]

Chapters are not processed when loading an ebook with an ncx file that uses a non-default namespace for ncx attributes/elements, and an Invalid argument supplied for foreach() warning is emitted.

Example bad toc.ncx:

<?xml version="1.0" encoding="utf-8" standalone="no"?>
<ncx:ncx xmlns:ncx="http://www.daisy.org/z3986/2005/ncx/" version="2005-1">
  <ncx:head>
    <ncx:meta name="cover" content="cover"/>
    <ncx:meta name="dtb:depth" content="-1"/>
    <ncx:meta name="dtb:totalPageCount" content="0"/>
    <ncx:meta name="dtb:maxPageNumber" content="0"/>
  </ncx:head>
  <ncx:docTitle>
    <ncx:text>Sample EBook</ncx:text>
  </ncx:docTitle>
  <ncx:navMap>
    <ncx:navPoint id="id286448480" playOrder="1">
      <ncx:navLabel>
        <ncx:text>Sample EBook</ncx:text>
      </ncx:navLabel>
      <ncx:content src="index.html"/>
      <ncx:navPoint playOrder="2" id="toc">
        <ncx:navLabel>
          <ncx:text>Table of Contents</ncx:text>
        </ncx:navLabel>
        <ncx:content src="toc.html"/>
      </ncx:navPoint>
      <ncx:navPoint id="id286448498" playOrder="3">
        <ncx:navLabel>
          <ncx:text>Sample EBook</ncx:text>
        </ncx:navLabel>
        <ncx:content src="pr01.html"/>
      </ncx:navPoint>
    </ncx:navPoint>
  </ncx:navMap>
</ncx:ncx>