Too much data for declared Content-Length when passing string with non-ascii characters via files parameter

[joshuahlang]

Checklist

• [x ] The bug is reproducible against the latest release and/or master.
• [ x] There are no similar issues or pull requests to fix it yet.

Describe the bug

h11._util.LocalProtocolError: Too much data for declared Content-Length is raised when passing string with non-ascii characters via files parameter.

To reproduce

import httpx

response = httpx.post(
    "https://httpbin.org/post",
    files={
        'upload-file': ('example.txt', '\u00E9', 'text/plain; charset=utf-8')
    }
)
response.raise_for_status()
print(response.read())

-->

Expected behavior

An exception not be raised and the content length properly computed

Actual behavior

An exception is raised.

Debugging material

Traceback (most recent call last):
  File "/***redacted***/httpx_bug.py", line 5, in <module>
    response = httpx.post(
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpx/_api.py", line 296, in post
    return request(
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpx/_api.py", line 93, in request
    return client.request(
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpx/_client.py", line 733, in request
    return self.send(
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpx/_client.py", line 767, in send
    response = self._send_handling_auth(
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpx/_client.py", line 805, in _send_handling_auth
    response = self._send_handling_redirects(
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpx/_client.py", line 837, in _send_handling_redirects
    response = self._send_single_request(request, timeout)
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpx/_client.py", line 861, in _send_single_request
    (status_code, headers, stream, ext) = transport.request(
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpcore/_sync/connection_pool.py", line 218, in request
    response = connection.request(
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpcore/_sync/connection.py", line 106, in request
    return self.connection.request(method, url, headers, stream, ext)
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpcore/_sync/http11.py", line 66, in request
    self._send_request_body(stream, timeout)
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpcore/_sync/http11.py", line 112, in _send_request_body
    self._send_event(event, timeout)
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpcore/_sync/http11.py", line 123, in _send_event
    bytes_to_send = self.h11_state.send(event)
  File "/***redacted***/.venv/lib/python3.9/site-packages/h11/_connection.py", line 468, in send
    data_list = self.send_with_data_passthrough(event)
  File "/***redacted***/.venv/lib/python3.9/site-packages/h11/_connection.py", line 501, in send_with_data_passthrough
    writer(event, data_list.append)
  File "/***redacted***/.venv/lib/python3.9/site-packages/h11/_writers.py", line 58, in __call__
    self.send_data(event.data, write)
  File "/***redacted***/.venv/lib/python3.9/site-packages/h11/_writers.py", line 78, in send_data
    raise LocalProtocolError("Too much data for declared Content-Length")
h11._util.LocalProtocolError: Too much data for declared Content-Length

-->

Environment

• OS: macOS
• Python version: Python 3.9.1
• HTTPX version: 0.16.1
• Async environment: n/a
• HTTP proxy: no
• Custom certificates: no

Additional context

n/a

[j178]

Looks like we calculate Content-Length using Unicode length:

httpx/httpx/_multipart.py

Lines 90 to 91 in 0f280af

	if isinstance(self.file, (str, bytes)):
	return len(headers) + len(self.file)

but sends with bytes:

httpx/httpx/_multipart.py

Lines 122 to 123 in 0f280af

	if isinstance(self.file, (str, bytes)):
	yield to_bytes(self.file)