Add an option for toFormData to allow null values

[maclementED]

I needed to send null values in a FormData. Sadly, there wasn't any way to indicate to keep null values as is.

This line is causing to transform null to empty string.

 function convertValue(value) {
    if (value === null) return '';
...

We would need to update options object and convertValue function to accept a parameter like nullsAsEmptyString

If needed, I can submit a PR for this feature