Ensure boolean value for isNewInstance

[rmg]

The tests are testing for true/false, not truthy/falsy, so the value
should be coerced to match.

@bajtos I found this while tinkering with loopbackio/loopback-connector-mongodb#116

[bajtos]

There are three valid values: true, false and undefined meaning either it's not possible to tell or that the connector does not provide the value yet. Which is the case of the mongodb connector I believe?

/cc @superkhau

[bajtos]

@rmg here is the algorithm I need to preserve:

• result or result.isNewInstance is not defined => ctx.isNewInstance = undefined
• result.isNewInstance is truthy => ctx.isNewInstance = true
• result.isNewInstance is false => ctx.isNewInstance = false

[rmg]

I made this change to match the test suite. I guess it's the test suite that needs updating instead.