use _.isDate to check if object is a Date object

[joliveros]

When running the snippet below, in some node versions the code fails because twilio-node is unable to format the given startDate and endDate parameters.

// Download the helper library from https://www.twilio.com/docs/node/install
// Your Account Sid and Auth Token from twilio.com/console
const accountSid = 'ACaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa';
const authToken = 'your_auth_token';
const client = require('twilio')(accountSid, authToken);

client.calls.feedbackSummaries
            .create({
               startDate: new Date(Date.UTC(2008, 0, 2)),
               endDate: new Date(Date.UTC(2008, 0, 2))
             })
            .then(feedback_summary => console.log(feedback_summary.sid))
            .done();

I've narrowed down the issue to the following line:
https://github.com/twilio/twilio-node/blob/master/lib/base/serialize.js#L22
This PR attempts to fix this issue.

[dprothero]

LGTM

[codejudas]

+1 Thanks for the fix!