Make fetchSlice compatible with custom repo method

If a user configures query_builder_method for the doctrine provider, and
does not alias the root entity to equal Provider::ENTITY_ALIAS then a
DQL error will occur during index population.

> [Semantical Error] [...] near 'a.id ASC': Error: 'a' is not defined.

Provider::countObjects already handles this by dynamically fetching the
alias with QueryBuilder::getRootAliases, and Provider::fetchSlice should
be doing the same.

nb. "Provider" refers to FOS\ElasticaBundle\Doctrine\ORM\Provider

Doctrine/ORM/Provider.php

@@ -9,7 +9,7 @@ use FOS\ElasticaBundle\Exception\InvalidArgumentTypeException;
 class Provider extends AbstractProvider
 {
     const ENTITY_ALIAS = 'a';
-    
+
     /**
      * @see FOS\ElasticaBundle\Doctrine\AbstractProvider::countObjects()
      */
@@ -50,12 +50,13 @@ class Provider extends AbstractProvider
          */
         $orderBy = $queryBuilder->getDQLPart('orderBy');
         if (empty($orderBy)) {
+            $rootAliases = $queryBuilder->getRootAliases();
             $identifierFieldNames = $this->managerRegistry
                 ->getManagerForClass($this->objectClass)
                 ->getClassMetadata($this->objectClass)
                 ->getIdentifierFieldNames();
             foreach ($identifierFieldNames as $fieldName) {
-                $queryBuilder->addOrderBy(static::ENTITY_ALIAS.'.'.$fieldName);
+                $queryBuilder->addOrderBy($rootAliases[0].'.'.$fieldName);
             }
         }