Description:
------------
Support responses with nested documents

Query Example: q=*:*&parentFilter=content_type:parentDocument&fq={!parent which=$parentFilter}&fl=*,[child parentFilter=$parentFilter]&start=0&rows=50

example response:
<response>

  <result name="response" numFound="3" start="0">
    <doc>
      <str name="id">parent_1</str>
      <doc>
        <str name="id">CHILD_1_1</str>
      </doc>
    </doc>
    <doc>
      <str name="id">parent_2</str>
      <doc>
        <str name="id">CHILD_2_1</str>
      </doc>
      <doc>
        <str name="id">CHILD_2_2</str>
      </doc>
    </doc>
    <doc>
      <str name="id">not_a_parent_1</str>
    </doc>
  </result>
</response>

Test script:
---------------
$fixtureXml = '<response>

  <result name="response" numFound="3" start="0">
    <doc>
      <str name="id">parent_1</str>
      <doc>
        <str name="id">CHILD_1_1</str>
      </doc>
    </doc>
    <doc>
      <str name="id">parent_2</str>
      <doc>
        <str name="id">CHILD_2_1</str>
      </doc>
      <doc>
        <str name="id">CHILD_2_2</str>
      </doc>
    </doc>
    <doc>
      <str name="id">not_a_parent_1</str>
    </doc>
  </result>
</response>';

$response = SolrUtils::digestXmlResponse($fixtureXml);

print_r($response);


Expected result:
----------------
SolrObject Object
(
    [response] => SolrObject Object
        (
            [numFound] => 3
            [start] => 0
            [docs] => Array
                (
                    [0] => SolrObject Object
                        (
                            [id] => parent_1
                            [_childDocuments_] => Array
                                (
                                    [0] => SolrObject Object
                                        (
                                            [id] => CHILD_1_1
                                        )

                                )

                        )

                    [1] => SolrObject Object
                        (
                            [id] => parent_2
                            [_childDocuments_] => Array
                                (
                                    [0] => SolrObject Object
                                        (
                                            [id] => CHILD_2_1
                                        )

                                    [1] => SolrObject Object
                                        (
                                            [id] => CHILD_2_2
                                        )

                                )

                        )

                    [2] => SolrObject Object
                        (
                            [id] => not_a_parent_1
                        )

                )

        )

)