Fix body of RpcV2CborClientPopulatesDefaultValuesInInput protocol test

[alextwoods]

Background

The previous body did not include the top level "default" member and decoded to:

{
  "defaultString"=>"hi", 
  "defaultBoolean"=>true, 
  "defaultList"=>[],
  # ect....
}

This adds in the top level "default" so the new body decodes as:

{
  "defaults"=>{
    "defaultString"=>"hi", 
    "defaultBoolean"=>true, 
    "defaultList"=>[],
    # ect...
  }
}

Testing

• Tested with smithy-ruby RPCV2 implementation.

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