Allow constructor property in Json

[klntsky]

> parseJsonStringToAeson "{\"constructor\": \"Just\"}"
(Left (TypeMismatch "JSON String"))

> parseJsonStringToAeson "{\"constructo\": \"Just\"}"  
(Right {"constructo":"Just"})

it comes from this library:

> jsonBigint.parse("{\"constructor\": \"Just\"}")

Uncaught:
{
  name: 'SyntaxError',
  message: 'Object contains forbidden constructor property',
  at: 16,
  text: '{"constructor": "Just"}'
}

We should probably initialize it with

{ constructorAction: 'preserve` }

[klntsky]

moved to mlabs-haskell/purescript-aeson#16