only check type for added documents to add query if provided as an array

library/Solarium/QueryType/Update/Query/Command/Add.php

@@ -89,11 +89,14 @@ class Add extends Command
      */
     public function addDocuments($documents)
     {
+        //only check documents for type if in an array (iterating a Traversable may do unnecessary work)
+        if (is_array($documents)) {
             foreach ($documents as $document) {
                 if (!($document instanceof DocumentInterface)) {
                     throw new RuntimeException('Documents must implement DocumentInterface.');
                 }
             }
+        }
 
         //if we don't have documents so far, accept arrays or Traversable objects as-is
         if (empty($this->documents)) {