Better check for date validity of contact.birthday / Fix: RangeError Invalid Date

The issue: on Safari & iOS webview if we perform a toJSON() on a instanceof Date object which is a 'Invalid Date' object, that throws an error. This does not happen on Firefox & Chrome!

I encountered this issue while sending device contacts over socket.io which iterates over contacts props and does a toJSON(), but the problem was that the Date object from birthday property was not valid.

Simple test case:

var contact = { birthday: null }; //data provided by cordova-plugin-contacts

try {
    contact.birthday = new Date(parseFloat(contact.birthday));
}
catch(e) {
   console.log(e); //this will never be called
}

//where the error occures..
contact.birthday.toJSON();

An 'Invalid Date' error does not throw anything, so a simple try-catch block doesn't pass the validity of that date.

The changes I made in convertUtils.js resolves this problem.