Swift container list call must explicitly specify JSON

https://stackoverflow.com/questions/35439056/jclouds-and-openstack-illegalstateexception-expected-begin-array-but-was-stri

https://github.com/jclouds/jclouds/blob/master/apis/openstack-swift/src/main/java/org/jclouds/openstack/swift/v1/features/ContainerApi.java

https://wiki.openstack.org/wiki/MetadataSearchAPI#Output_formats

http://developer.openstack.org/api-ref-objectstorage-v1.html

The container list calls, in addition to the header "Accept: application/json" (which seems to be sufficient for some providers) are required by the spec to also send ?format=json in the URL. Not doing that seems to cause issues.