title: How to use xpath from lxml on null namespaced nodes?
body: <p>What is the best way to handle the lack of a namespace on some of the nodes in an xml document using lxml?  Should I first modify all None named nodes to add the "gmd" name and then change the tree attributes to name <a href="http://www.isotc211.org/2005/gmd" rel="noreferrer">http://www.isotc211.org/2005/gmd</a> as "gmd"?  If so, is there a clean way to do this with lxml or something else that would be relatively clean/safe?</p>
  
  <pre><code>from lxml import etree
  nsmap = charts_tree.nsmap
  nsmap.pop(None) # complains without this on the xpath with
  # TypeError: empty namespace prefix is not supported in XPath
  len (charts_tree.xpath('//*/gml:Polygon',namespaces=nsmap))
  # 1180
  len (charts_tree.xpath('//*/DS_DataSet',namespaces=nsmap))
  # 0 ... Bummer!
  len (charts_tree.xpath('//*/DS_DataSet'))
  # 0 ... Also a bummer
  </code></pre>
  
  <p>e.g. <a href="http://www.charts.noaa.gov/ENCs/ENCProdCat_19115.xml" rel="noreferrer">http://www.charts.noaa.gov/ENCs/ENCProdCat_19115.xml</a></p>
  
  <pre><code>&lt;DS_Series xmlns="http://www.isotc211.org/2005/gmd" xmlns:gco="http://www.isotc211.org/2005/gco" xmlns:gml="http://www.opengis.net/gml/3.2" xmlns:gsr="http://www.isotc211.org/2005/gsr" xmlns:gss="http://www.isotc211.org/2005/gss" xmlns:gts="http://www.isotc211.org/2005/gts" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.isotc211.org/2005/gmd http://schemas.opengis.net/iso/19139/20070417/gmd/gmd.xsd"&gt;
  &lt;composedOf&gt;
      &lt;DS_DataSet&gt;
          &lt;has&gt;
              &lt;MD_Metadata&gt;
                  &lt;parentIdentifier&gt;
                      &lt;gco:CharacterString&gt;NOAA ENC Product Catalog&lt;/gco:CharacterString&gt;
                  &lt;/parentIdentifier&gt;
  ...
  &lt;EX_BoundingPolygon&gt;
      &lt;polygon&gt;
          &lt;gml:Polygon gml:id="US1AK90M_P1"&gt;
              &lt;gml:exterior&gt;
                  &lt;gml:LinearRing&gt;
                      &lt;gml:pos&gt;67.61505 -178.99979&lt;/gml:pos&gt;
                      &lt;gml:pos&gt;73.99999 -178.99979&lt;/gml:pos&gt;
  ...
                      &lt;gml:pos&gt;64.99997 -178.99979&lt;/gml:pos&gt;
                      &lt;gml:pos&gt;67.61505 -178.99979&lt;/gml:pos&gt;
                  &lt;/gml:LinearRing&gt;
  </code></pre>