static _serializeNumericString(form, serializedSpaceCharacter = '+') {
        return this._serialize(form, false, 'unformatted', serializedSpaceCharacter);
    }
Serialize the form values to a string, outputting numeric strings for each AutoNumeric-managed element values.

@param {HTMLFormElement} form
@param {string} serializedSpaceCharacter
@returns {string}