lfDraw edit attributes gets overwritten when initial featureGroup is undefined

[eugenehuangsg]

lfDraw currently supports adding of featureGroup but there is an issue.

Given a configuration

edit: {
    edit: true,
    remove: false
}

Expected: (adding featureGroup)

edit: {
    edit: true,
    remove: false
    featureGroup: new L.FeatureGroup()
}

Current: (replacing the attributes entirely)

edit: {
    featureGroup: new L.FeatureGroup()
}