Fix DynamoDB DocumentClient set unmarshalling

[srchase]

• npm run test passes
• changelog is added, npm run add-change

Using the DynamoDB DocumentClient, a set within an item will get stringified improperly:

const AWS = require('aws-sdk');
const client = new AWS.DynamoDB.DoclumentClient();
const params = {
  TableName: 'myTable',
  Key: {
    id: 1
  }
}
client.get(params, function(err,data) {
  if (!err) {
    console.log(JSON.stringify(data)); // {"Item":{"id":1,"list1":{"wrapperName":"Set","values":[1,5,9],"type":"Number"}}}
  }
}

Instead, only the values of the set should be returned:

console.log(JSON.stringify(data)); // {"Item":{"id":55,"list1":[1,5,9]}}

[codecov-io]

Codecov Report

Merging #2647 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2647      +/-   ##
==========================================
+ Coverage   96.83%   96.83%   +<.01%     
==========================================
  Files         283      283              
  Lines        8615     8618       +3     
  Branches     1640     1640              
==========================================
+ Hits         8342     8345       +3     
  Misses        273      273

Impacted Files	Coverage Δ
lib/dynamodb/set.js	100% <100%> (ø)	⬆️

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[AllanZhengYP]

Looks good to me 🚢

[lock]

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