add print_job data

[dellorogiulio]

Hi @ctalkington, seems I figured out how to print a document using this library. I found a missing part comparing your source with sealsystems/node-ipp.

In sealsystems lib, they used a 'custom' data attribute in which they set the file data (see this example) and then they add the 'already-byte' data directly to the encoded message.

so in your `serializer.py', it is needed a

encoded +=  data['data']

just before returning encoded.

In my test, a pdf file is correctly printed.

Let me know if you intend to update this library or if I need to use my fork (i need to use this library without patching the source)