Accept-Encoding header in PDF printing example

[andybalholm]

The example showing how to print a PDF file sets an Accept-Encoding header on the HTTP request, but that doesn't work correctly. If the printer returns a compressed response, decoding the IPP response fails. (IPP decode: Invalid tag 0 at 0x8) If we leave that header off, the Go HTTP package puts in an accept-encoding header transparently, and everything works.