Support appending pre-encoded data when serializing

[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)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 94.49%. Comparing base (e626e4e) to head (8b658f9).

Files	Patch %	Lines
src/pyipp/serializer.py	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #510      +/-   ##
==========================================
- Coverage   94.71%   94.49%   -0.23%     
==========================================
  Files           9        9              
  Lines         851      853       +2     
  Branches      130      131       +1     
==========================================
  Hits          806      806              
- Misses         19       20       +1     
- Partials       26       27       +1     

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