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
Merge pull request #510 from peterjmit/master

Make fetchSlice compatible with custom repo method