Put index name in URL for _bulk operations, if it is supplied

At present, _bulk operations are POSTed to /_bulk, regardless of if an index name is specified or not.  This change causes these operations, when an index is specified, to POST to /indexname/_bulk.

Use-case: ElasticSearch is behind a proxy that requires authentication that differs per-index.  Some NEST operations put the index name in the URL, but some others (at least _bulk) never do, which breaks this security model.