title: Mocking AWS with dwyl/aws-sdk-mock not working
body: <p>First of all, I'm not using lambda, so I believe <a href="https://stackoverflow.com/questions/38486029/aws-mock-sdk-s3-getobject-not-working">this thread</a> isn't the same question.</p>
  
  <p>I'm getting this error in the <code>before</code> hook:</p>
  
  <pre><code>TypeError: Attempted to wrap undefined property S3 as function
        at checkWrappedMethod (node_modules/aws-sdk-mock/node_modules/sinon/lib/sinon/util/core.js:78:29)
        at Object.wrapMethod (node_modules/aws-sdk-mock/node_modules/sinon/lib/sinon/util/core.js:121:21)
        at Object.stub (node_modules/aws-sdk-mock/node_modules/sinon/lib/sinon/stub.js:67:26)
        at mockService (node_modules/aws-sdk-mock/index.js:67:27)
        at Object.AWS.mock (node_modules/aws-sdk-mock/index.js:43:5)
        at Context.&lt;anonymous&gt; (myModule.spec.js:14:9)
  </code></pre>
  
  <p>I've never used this module before neither do I have much experience with sinon.js.</p>
  
  <p>Here is my module:</p>
  
  <pre><code>var AWS = require('aws-sdk')
  const https = require('https')
  
  module.exports = function () {
      var s3 = new AWS.S3({
        httpOptions: {
          agent: new https.Agent()
        },
        signatureVersion: 'v4'
      })
      return s3.getObject({Bucket: 'somebucket', Key: 'somekey'}).promise()
  }
  </code></pre>
  
  <p>Here is my test script:</p>
  
  <pre><code>const chai = require('chai')
  const chaiAsPromised = require('chai-as-promised')
  
  const AWS = require('aws-sdk-mock')
  const myFunc = require('./myModule')
  
  chai.use(chaiAsPromised)
  const expect = chai.expect
  chai.should()
  
  describe('Get MMS API Key', function () {
  
    before(function () {
      AWS.mock('S3', 'getObject', function (params, callback) {
        callback(null, 'dummy-data')
      })
    })
  
    it('Should get dummy data', function () {
      myFunc().should.eventually.equal('dummy-data')
    })
  
    after(function () {
      AWS.restore('S3', 'getObject')
    })
  })
  </code></pre>
  
  <p>Thoughts?</p>