Request.send chokes when not provided a callback

[tmonoceros]

The fix for this issue breaks some previously-functioning tests.

The callback provided to request.send() is optional in the AWS SDK. Starting with 5.3, invoking send() on a mocked request without a callback causes a crash and thus test failures.

This can be worked around but can be annoying and does make 5.3 a breaking release.

Example:

const awsMock = require('aws-sdk-mock')
const aws = require('aws-sdk')

awsMock.mock('EventBridge', 'putEvents')
const eb = new aws.EventBridge()

const request = eb.putEvents('stuff')
request.send() //crash in 5.3+, fine in 5.2
awsMock.restore()