Fix Select Component MoreLikeThis to support boosting multiple fields

library/Solarium/QueryType/Select/RequestBuilder/Component/MoreLikeThis.php

@@ -68,7 +68,7 @@ class MoreLikeThis implements ComponentRequestBuilderInterface
         $request->addParam('mlt.boost', $component->getBoost());
         $request->addParam(
             'mlt.qf',
-            count($component->getQueryFields()) ? implode(',', $component->getQueryFields()) : null
+            count($component->getQueryFields()) ? $component->getQueryFields() : null
         );
         $request->addParam('mlt.count', $component->getCount());
 

tests/Solarium/Tests/QueryType/Select/RequestBuilder/Component/MoreLikeThisTest.php

@@ -67,7 +67,7 @@ class MoreLikeThisTest extends \PHPUnit_Framework_TestCase
                 'mlt.maxqt' => 4,
                 'mlt.maxntp' => 5,
                 'mlt.boost' => 'true',
-                'mlt.qf' => 'description',
+                'mlt.qf' => array('description'),
                 'mlt.count' => 6,
             ),
             $request->getParams()