Fix for JCLOUDS-655

First time committing to jclouds, let me know if there are any style guidelines I missed with this change.
Makes sense to me, but I might suggest a different field name for future upgrades (never know when another object will show up in there). Perhaps `complexMetadata`? 

Either way, I'll need to dig into the deserialization a bit. Any pointers on where to start looking for examples in the code base?