Padding option should allow zero values

[sevrai]

Hello,

I noticed padding parameters omit zero values when I use generateImageUrl method.
The resulting images are not the same :

/insecure/pd:50:0:50:0/bg:FFFF00/rs:fit:500/plain/path	/insecure/pd:50::50/bg:FFFF00/rs:fit:500/plain/path

The option object that can be used for reproduction is the following:

const options = {
  padding: {
    left: 0
    top: 50
    right: 0
    bottom: 50
  }
}

I'll create a PR with the fix 🙂