plainAddPlaceholder utf8 support

[tomasantunes]

I need utf8 support on the placeholders. When I insert a name on the placeholder I get garbled text on the final signature.

Here is the code I'm using:

  let inputBuffer = plainAddPlaceholder({
      pdfBuffer,
      reason: 'Signed Certificate.',
      contactInfo: '[email protected]',
      name: 'JOÃO SILVA',
      location: 'Porto',
      signatureLength: totalLength,
  });

Here is how it looks on the signature: