title: Retain namespace prefix in a tag when parsing xml using lxml
body: <p>I have an xml as below. There are few tags which are prefixed with <code>ce</code> for example <code>&lt;ce:title&gt;</code>. When I run the code as below with xpath, in output, <code>&lt;ce:title&gt;</code> is replaced with <code>&lt;title&gt;</code>. I did see other links on SO like <a href="https://stackoverflow.com/questions/6597271/how-to-preserve-namespace-information-when-parsing-html-with-lxml">How to preserve namespace information when parsing HTML with lxml?</a> but not sure where and how to add namespace details.</p>
  <p>Can someone please suggest ? How can I retain <code>&lt;ce:title&gt;</code> for below xml?</p>
  <pre><code>from lxml import html
  from lxml.etree import tostring
  with open('102277033304.xml', encoding='utf-8') as file_object:
      xml = file_object.read().strip()
      root = html.fromstring(xml)
      for element in root.xpath('//item/book/pages/*'):
          html = tostring(element, encoding='utf-8')
          print(html)
  </code></pre>
  <p>XML:</p>
  <pre><code>&lt;item&gt;
      &lt;book&gt;
          &lt;pages&gt;
              &lt;page-info&gt;
                  &lt;page&gt;
                    &lt;ce:title&gt;Chapter 1&lt;/ce:title&gt;
                    &lt;content&gt;Welcome to Chapter 1&lt;/content&gt;
                  &lt;/page&gt;
                  &lt;page&gt;
                   &lt;ce:title&gt;Chapter 2&lt;/ce:title&gt;
                   &lt;content&gt;Welcome to Chapter 2&lt;/content&gt;
                  &lt;/page&gt;
              &lt;/page-info&gt;
              &lt;page-fulltext&gt;Published in page 1&lt;/page-fulltext&gt;
              &lt;page-info&gt;
                  &lt;page&gt;
                    &lt;ce:title&gt;Chapter 1&lt;/ce:title&gt;
                    &lt;content&gt;Welcome to Chapter 1&lt;/content&gt;
                  &lt;/page&gt;
                  &lt;page&gt;
                   &lt;ce:title&gt;Chapter 2&lt;/ce:title&gt;
                   &lt;content&gt;Welcome to Chapter 2&lt;/content&gt;
                  &lt;/page&gt;
              &lt;/page-info&gt;
              &lt;page-fulltext&gt;Published in page 2&lt;/page-fulltext&gt;
              &lt;page-info&gt;
                  &lt;page&gt;
                    &lt;ce:title&gt;Chapter 1&lt;/ce:title&gt;
                    &lt;content&gt;Welcome to Chapter 1&lt;/content&gt;
                  &lt;/page&gt;
                  &lt;page&gt;
                   &lt;ce:title&gt;Chapter 2&lt;/ce:title&gt;
                   &lt;content&gt;Welcome to Chapter 2&lt;/content&gt;
                  &lt;/page&gt;
              &lt;/page-info&gt;
              &lt;page-fulltext&gt;Published in page 3&lt;/page-fulltext&gt;
          &lt;/pages&gt;
      &lt;/book&gt;
  &lt;/item&gt;
  </code></pre>