Fix errors in OPENER client.

- Content-Type set to text/xml with charset=UTF-8.
- Void calls now return SetInterfaceResponse.
- addInterface call set to POST.
- Fix IPRequest prefix lenght element. Use <prefixlen> tag instead of <prefix-length>.
To achieve that, IPRequest now uses IPDataRequest instead of IPData.

Test updated to match changes in the interface.

SetInterfaceIPAddress and DeleteInterfaceIPAddress calls do work.