Omit undefined values in stringify

[RikSchefferAmsterdam]

This PR makes sure undefined values are omitted when using stringify.

Solves: #46

For example:

const obj = {
      a: undefined,
      b: 'bar'
};
console.log(stringify(obj)) // 'b=bar'

[43081j]

makes sense to me! this is a bug for sure

thanks for the contribution 💙