`properties` throws `RangeError: Invalid time value` when the error object has field of type `Date` with value `Invalid Date`

[nedyalkov]

If you call the properties method on an error object that has a field of type Date with a value Invalid Date, it tries to call the toISOString method of this object and throws an error: RangeError: Invalid time value.

To reproduce this, do the following:

var stackman = require('stackman')()

var err = new Error('Oops!')
err.number = Number.NaN;
err.date = new Date('invalid')

var props = stackman.properties(err)
console.log(props)

If you comment out err.date = new Date('invalid') or change the value to a valid date, the code works.