title: jQuery .attr() returns "undefined" for existing attribute
body: <p>I have been trying to access a custom attribute defined on a <code>svg:rect</code> DOM element using jQuery (v2.1.4). The exact element is below:</p>
  
  <pre><code>&lt;rect id="lane2-diag_D" currentLane="2" class="draggableCircle" x="250" y="125" height="20" width="10" clr="Chartreuse" style="stroke: gray; stroke-width: 1px; fill: chartreuse;"&gt;&lt;/rect&gt;
  </code></pre>
  
  <p>This element is cloned from another element and it's id is changed. I'm trying to access the attribute after adding it to an <code>svg</code> element on the page.</p>
  
  <p>When I try to access the <code>currentLane</code> property using <code>$('#lane2-diag_D').attr('currentLane')</code> it returns <code>undefined</code>. If I try to access  it with the javascript approach <code>document.getElementById('lane2-diag_D').getAttribute('currentLane')</code> it returns the correct value.</p>
  
  <p>For all other element attributes jQuery returns their correct values.</p>
  
  <p>What am I missing in the jQuery syntax?</p>