<p>This PR refactors the trace api_update method to work on existing trace objects, rather than creating temporary 'shadow' trace objects. In doing so it refactors the <code>from_xml</code> and <code>from_xml_node</code> model methods to be instance methods instead of class methods.</p>
<p>Advantages:</p>
<ul>
<li>No longer duplicate tags (i.e. <span class="issue-keyword tooltipped tooltipped-se" aria-label="This pull request closes issue #1600.">fixes</span> <a href="https://github.com/openstreetmap/openstreetmap-website/issues/1600" class="issue-link js-issue-link" data-url="https://github.com/openstreetmap/openstreetmap-website/issues/1600" data-id="246895755" data-error-text="Failed to load issue title" data-permission-text="Issue title is private">#1600</a>)</li>
<li>makes updating a trace via xml more similar to updating via forms (which don't use shadow objects)</li>
</ul>
<p>Concerns:</p>
<ul>
<li>Doesn't avoid deleting and recreating tag objects when the tags are the same (but neither does the <code>tagstring=</code> method, afaict)</li>
<li>Other classes (Node, Changeset etc) all use the class method approach for <code>from_xml</code>, rather than instance methods. I don't know whether they should be refactored too, or whether there's some advantage of the class method approach that I haven't understood.</li>
</ul>

<hr>

<h4>You can view, comment on, or merge this pull request online at:</h4>
<p>  <a href='https://github.com/openstreetmap/openstreetmap-website/pull/1614'>https://github.com/openstreetmap/openstreetmap-website/pull/1614</a></p>

<h4>Commit Summary</h4>
<ul>
  <li>Add failing test demonstrating tag duplication when updating traces via the API.</li>
  <li>Refactor the from_xml methods to act on existing trace objects.</li>
</ul>

<h4>File Changes</h4>
<ul>
  <li>
    <strong>M</strong>
    <a href="https://github.com/openstreetmap/openstreetmap-website/pull/1614/files#diff-0">app/controllers/trace_controller.rb</a>
    (10)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/openstreetmap/openstreetmap-website/pull/1614/files#diff-1">app/models/trace.rb</a>
    (26)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/openstreetmap/openstreetmap-website/pull/1614/files#diff-2">test/controllers/trace_controller_test.rb</a>
    (17)
  </li>
</ul>

<h4>Patch Links:</h4>
<ul>
  <li><a href='https://github.com/openstreetmap/openstreetmap-website/pull/1614.patch'>https://github.com/openstreetmap/openstreetmap-website/pull/1614.patch</a></li>
  <li><a href='https://github.com/openstreetmap/openstreetmap-website/pull/1614.diff'>https://github.com/openstreetmap/openstreetmap-website/pull/1614.diff</a></li>
</ul>

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