Improved rest_server XML formatter for numeric arrays

For our use case, rest_server's XML formatter didn't output entirely accurate XML. Numeric arrays should be formatted as repeated tags, rather than inside "item" tags.

For example, if you have custom values in $node, something like this:

$node->component['attribute'][0]['value'] = 'test0';
$node->component['attribute'][1]['value'] = 'test1';

The current XML formatter will display something like this:

<component>
  <attribute is_array="true">
    <item>
      <value>test0</value>
    </item>
    <item>
      <value>test1</value>
    </item>
  </attribute>
</component>

Whereas it "should" (at least for our use case) be like this:

<component>
  <attribute>
    <value>test0</value>
  </attribute>
  <attribute>
    <value>test1</value>
  </attribute>
</component>

The main reason this is important for us is because we're uploading XML files using a custom solution and turning them into nodes on the backend. We then need to generate exactly the same XML format for API queries.

Attached is a patch that accomplishes this. It does currently work for us, but I'm leaving this as "needs work" since I'm not sure how it may impact other people's implementations.

Thanks!