Add tests for nextPage override.

Fixes #711

test/dynamodb/document_client.coffee

@@ -581,3 +581,56 @@ describe 'AWS.DynamoDB.DocumentClient', ->
       docClient.get {Key: foo: 1}, (err, data) ->
         expect(data).to.eql(output)
         done()
+
+  describe 'response.nextPage', ->
+
+    client = null; response = null; request = null
+
+    beforeEach ->
+      client = new AWS.DynamoDB.DocumentClient({paramValidation: false})
+      request = client.query({ExpressionAttributeValues: { foo: 'bar' }})
+      response = request.response
+
+    fill = (err, data) ->
+      request.emit('validate', [request])
+      response.error = err
+      response.data = data
+
+    it 'returns null if there are no more pages', ->
+      fill(null, {})
+      expect(response.nextPage()).to.equal(null)
+
+    it 'returns a request object with the next page marker filled in params', ->
+      fill(null, LastEvaluatedKey: { fizz: 'pop' })
+      req = response.nextPage()
+      expect(req.params.ExclusiveStartKey.fizz).to.equal('pop')
+      expect(req.operation).to.equal(response.request.operation)
+
+    it 'uses untranslated params', ->
+      fill(null, LastEvaluatedKey: 'baz')
+      req = response.nextPage()
+      expect(req.params.ExpressionAttributeValues.foo).to.equal('bar')
+
+    it 'throws error if response returned an error and there is no callback', ->
+      fill(new Error('error!'), null, true)
+      expect(-> response.nextPage()).to.throw('error!')
+
+    it 'sends the request if passed with a callback', (done) ->
+      helpers.mockHttpResponse 200, {}, ['']
+      fill(null, LastEvaluatedKey: 'baz')
+      response.nextPage (err, data) ->
+        expect(err).to.equal(null)
+        expect(data).to.eql({})
+        done()
+
+    it 'passes null to callback if there are no more pages', ->
+      fill(null, {})
+      response.nextPage (err, data) ->
+        expect(err).to.equal(null)
+        expect(data).to.equal(null)
+
+    it 'passes error through if original response returned an error', ->
+      fill('error!', null)
+      response.nextPage (err, data) ->
+        expect(err).to.equal('error!')
+        expect(data).to.equal(null)