Registration fails "An access token should not be provided on requests to /register"

Describe the bug
Trying to register a new account fails with "An access token should not be provided on requests to /register (except if type is m.login.application_service)", although no token was ever set.

To Reproduce

from matrix_client.client import MatrixClient
matrix = MatrixClient("${homeserverUrl}")
matrix.register_with_password(username="alice", password="foobar")

Expected behavior
Login works fine

Actual behavior

Client side:

Traceback (most recent call last):
  File "test.py", line 3, in <module>
    matrix.register_with_password(username="alice", password="foobar")
  File "/lib/python3.9/site-packages/matrix_client/client.py", line 203, in register_with_password
    response = self.api.register(
  File "/lib/python3.9/site-packages/matrix_client/api.py", line 160, in register
    return self._send(
  File "/lib/python3.9/site-packages/matrix_client/api.py", line 748, in _send
    raise MatrixRequestError(
matrix_client.errors.MatrixRequestError: 400: {"errcode":"M_UNKNOWN","error":"An access token should not be provided on requests to /register (except if type is m.login.application_service)"}

Server side:

[  104.741510] synapse[1153]: synapse.http.server: [POST-1] <SynapseRequest at 0x7fc64ed887f0 method='POST' uri='/_matrix/client/r0/register?kind=user' clientproto='HTTP/1.1' site='8448'> SynapseError: 400 - An access token should not be provided on requests to /register (except if type is m.login.application_service)
[  104.753403] synapse[1153]: synapse.access.http.8448: [POST-1] ::ffff:192.168.1.2 - 8448 - {None} Processed request: 0.010sec/-0.000sec (0.001sec, 0.001sec) (0.000sec/0.000sec/0) 140B 400 "POST /_matrix/client/r0/register?kind=user HTTP/1.1" "matrix-python-sdk/0.4.0" [0 dbevts]

Additional context
This is probably a regression in 0.4.0 related to the addition of use_authorization_header. Also the tests currently only test the creation of guest accounts.

Start in matrix_client/api.py at the _send and register calls shown in the traceback, then review matrix_client/client.py's register_with_password path and the existing guest-account tests. Reproduce the registration request and confirm it succeeds without an access token; add coverage for password registration alongside the current tests.