Solves #2813.

Referring to #2813, subsequent <Document /> renders produces a PDF file with non-deterministic /ID even though the creationDate and modificationDate props were set. If I managed to understand correctly, the ID is generated when the PDFDocument class was instantiated. However, some of the .info object's fields were updated from outside the class instance during the rendering process:

const setPDFMetadata = (target) => (key, value) => {
  if (value) target.info[key] = value;
};

const addMetadata = (ctx, doc) => {
  const setProp = setPDFMetadata(ctx);

  // ...
  const creationDate = props.creationDate || new Date();
  const modificationDate = props.modificationDate || null;

  // ...
  setProp('CreationDate', creationDate);
  setProp('ModificationDate', modificationDate);
};

However, the ID was generated based on the .info in the constructor and was not updated after the .info object was updated. This means that the ID is generated with the default CreationDate which is the current time.

// Initialize the metadata
this.info = {
  Producer: 'PDFKit',
  Creator: 'PDFKit',
  CreationDate: new Date()
};

if (this.options.info) {
  for (let key in this.options.info) {
    const val = this.options.info[key];
    this.info[key] = val;
  }
}

this._id = PDFSecurity.generateFileID(this.info);

This PR changes the behaviour so that the ID is not generated in the constructor but rather only when it is needed. Therefore, the modifications in the .info object should already be made at that point.