Fixed retrieving of 'ipAddress' attribute of VMs inside VAPP

Retrieving of a namespaced attribute doesn't work unless you specify the attribute name with complete namespace ahead.

That's the original xml snippet of vcloud rest API answer:

```
<ovf:Item>
...
   <rasd:Connection vcloud:ipAddress="MY_IPADDRESS" 
      vcloud:primaryNetworkConnection="true" 
      vcloud:ipAddressingMode="POOL">MY_NETWORK_CONNECTION</rasd:Connection>
...
</ovf:Item>
```

Inside code, this one works:
```
n['vcloud:ipAddress']
```
this one doesn't:
```
n[ipAddress']
```

Environment:
 * nokogiri (1.5.9)
 * ruby 1.9.3p385 (2013-02-06 revision 39114) [x86_64-linux]
 * ruby 1.9.3p327 (2012-11-10 revision 37606) [x86_64-linux]