Request builds incorrectly if data values are passed as bytes

[harrylloyd-bl]

OS platform: Windows
Python version: 3.12.0
Installed dependencies and versions (python -m pip freeze):
Conda environment: conda_env.log
httpx - 0.27.0

Code snippet

data = {  # sample data with bytes for the oauth_body_hash and oauth_signature
    'oauth_body_hash': b'myoauthbodyhash',  # bytes
    'oauth_signature': b'myoauthsignature'  # bytes
}


with httpx.Client() as c:
    req = c.build_request(
        method="POST",
        url="https://example.org",
        data=data
    )

    # break the request back down
    deconstruct_req = req.content.decode("utf-8").split("&")
    deconstructed_params = {p.split("=")[0]: p.split("=")[1] for p in deconstruct_req}

    assert deconstructed_params["oauth_signature"] == data["oauth_signature"]
    assert deconstructed_params["oauth_body_hash"] == data["oauth_body_hash"]

Error traceback
This fails because the oauth_signature and oauth_body_hash have the b'' byte object markers included when the body of the url is encoded

• Result: req.content = b"oauth_body_hash=b%27myoauthbodyhash%27&oauth_signature=b%27oauthsignature%27"
• Expected result: req.content = b"oauth_body_hash=myoauthbodyhash&oauth_signature=myoauthsignature"

Does the issue exist on HTTP/1.1, or HTTP/2, or both? HTTP/1.1, unsure about /2
Does the issue exist with Client, AsyncClient, or both? Both
When using AsyncClient does the issue exist when using asyncio or trio, or both? Both

This is loosely related to #3115

This will apply to any parameter in data, I came across it because the oauth lib I'm using returns the signature and body hash as bytes. If it's best practice that data should be passed as str not bytes then I'm happy to have this closed, but the RequestData typing is currently Mapping[str, Any], so that might be updated to exclude bytes.

The issue arises because Request calls _content.encode_urlencoded_data which uses _utils.primitive_value_to_str to stringify the values in data. _utils.primitive_value_to_str takes type PrimitiveData = Optional[Union[str, int, float, bool]], so isn't expecting bytes. Aside from decoding any bytes values in data before they're passed to build_request the fix that I've used is to add an elif to _utils.primitive_value_to_str to return the utf-8 decoded string if type == bytes. The typing for _utils.primitive_value_to_str would need to change though.