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

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

@@ -89,9 +89,12 @@ class Add extends Command
      */
     public function addDocuments($documents)
     {
-        foreach ($documents as $document) {
-            if (!($document instanceof DocumentInterface)) {
-                throw new RuntimeException('Documents must implement DocumentInterface.');
+        //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.');
+                }
             }
         }