Fix `getaddednodeinfo` RPC call with dns=false

The getaddednodeinfo RPC call, when invoked with the dns flag set to
false, returns a malformed JSON object with duplicate keys.

Change this to return an array of objects with one key as
shown in the help message.

Fixes #3581.
Merge pull request #3586

4412c5a Fix `getaddednodeinfo` RPC call with dns=false (Wladimir J. van der Laan)