Fix `getaddednodeinfo` RPC call with dns=false
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.

Rebased-from: a41825be5762ff36dcf0d89c20b95572af30c6bb 0.8.x

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