[py3] bug.lp_save() crashes with TypeError: startswith first arg must be bytes or a tuple of bytes, not str

While porting Apport to python3-launchpadlib, I stumbled over the inability to call lp_save() on a bug object:

$ python3 /tmp/test.py
Traceback (most recent call last):
  File "/tmp/test.py", line 10, in <module>
    bug.lp_save()
  File "/usr/lib/python3/dist-packages/lazr/restfulclient/resource.py", line 778, in lp_save
    URI(self.self_link), representation, headers)
  File "/usr/lib/python3/dist-packages/lazr/restfulclient/_browser.py", line 475, in patch
    cached_etag = self._connection._getCachedHeader(str(url), 'etag')
  File "/usr/lib/python3/dist-packages/lazr/restfulclient/_browser.py", line 188, in _getCachedHeader
    return self.cache._getCachedHeader(uri, header)
  File "/usr/lib/python3/dist-packages/lazr/restfulclient/_browser.py", line 323, in _getCachedHeader
    if line.startswith(header_start):
TypeError: startswith first arg must be bytes or a tuple of bytes, not str

When I call the same with python 2, it works fine. Note that this is a write operation, so this needs to use login_with() and credentials. So you might want to change the credentials_file= argument in the reproducer or drop it completely (then you'll have to log in every time you run the reproducer).