First item in a !!pair is unnecessarily converted into string

[rlidwka]

pairs:
  Block tasks: !!pairs
    - { test: 123 }: { test: 123 }
    - { test: 456 }: { test: 456 }

{ pairs: 
   { 'Block tasks': 
      [ [ '[object Object]', { test: 123 } ],
        [ '[object Object]', { test: 456 } ] ] } }

In normal maps { test: 123 } is turned into string because javascript doesn't support objects as keys (Maps aside, as we don't support those).

But pairs are turned into arrays, which can have objects inside. So stringification is unnecessary there.

somewhat related to #169, but this is an issue that can be fixed without rewriting everything