rgw: fix swift cannot disable object versioning

we should be able to disable object verioning by removing its X-Versions-Location
metadata header by sending an empty key value. this description can be found at
No.8 in http://docs.openstack.org/user-guide/cli-swift-set-object-versions.html.

Fixes: http://tracker.ceph.com/issues/18852
Signed-off-by: Jing Wenjun <jingwenjun@cmss.chinamobile.com>

src/rgw/rgw_rest_swift.cc

@@ -604,7 +604,7 @@ static int get_swift_versioning_settings(
   }
 
   std::string vloc = s->info.env->get("HTTP_X_VERSIONS_LOCATION", "");
-  if (vloc.size()) {
+  if (s->info.env->exists("HTTP_X_VERSIONS_LOCATION")) {
     /* If the Swift's versioning is globally disabled but someone wants to
      * enable it for a given container, new version of Swift will generate
      * the precondition failed error. */