RFC822 writer is flaky

The RFC822 writer should not use % (or use it correctly)

CRITICAL plainbox.crashes: Plik wykonywalny 'plainbox' uruchomiony z Namespace(command=<plainbox.impl.commands.run.RunCommand object at 0x7f6d1a4475c0>, debug_console=False, debug_interrupt=False, dry_run=False, exclude_pattern_list=[], include_pattern_list=['2013.com.canonical.certification::graphics/generator_3d_window_move', '2013.com.canonical.certification::graphics/1_3d_window_move_.+'], log_level=None, not_interactive=False, output_file=<_io.TextIOWrapper name='<stdout>' mode='w' encoding='UTF-8'>, output_format='rfc822', output_options='with-io-log', pdb=False, providers=None, trace=[], transport=None, transport_options=None, transport_where=None, whitelist=[]) uległ awarii
Traceback (most recent call last):
  File "/home/zyga/checkbox/plainbox/plainbox/impl/clitools.py", line 529, in dispatch_and_catch_exceptions
    return self.dispatch_command(ns)
  File "/home/zyga/checkbox/plainbox/plainbox/impl/clitools.py", line 525, in dispatch_command
    return ns.command.invoked(ns)
  File "/home/zyga/checkbox/plainbox/plainbox/impl/commands/run.py", line 623, in invoked
    return RunInvocation(self.provider_list, self.config, ns).run()
  File "/home/zyga/checkbox/plainbox/plainbox/impl/commands/run.py", line 205, in run
    self.export_and_send_results()
  File "/home/zyga/checkbox/plainbox/plainbox/impl/commands/run.py", line 466, in export_and_send_results
    self.exporter.dump(data_subset, exported_stream)
  File "/home/zyga/checkbox/plainbox/plainbox/impl/exporter/rfc822.py", line 47, in dump
    RFC822Record(entry).dump(string_stream)
  File "/home/zyga/checkbox/plainbox/plainbox/impl/secure/rfc822.py", line 331, in dump
    _dump_part(stream, key, value)
  File "/home/zyga/checkbox/plainbox/plainbox/impl/secure/rfc822.py", line 328, in _dump_part
    stream.write(" %s\n" % value)
TypeError: not all arguments converted during string formatting