RedirectResponse call to `quote_plus` replaces spaces & `%20`'s with `+` character

Checklist

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

Describe the bug

RedirectResponse replaces url paths with spaces or %20 with the + character. This happens as the location header is written after passing the url to quote_plus: https://github.com/encode/starlette/blob/master/starlette/responses.py#L181

If part of the path was a path param (with a space in it), this then modified those path params.

We currently encounter this with trailing slash redirects, which return a RedirectResponse.

To reproduce

from starlette.applications import Starlette
from starlette.responses import JSONResponse
from starlette.routing import Route


async def homepage(request):
    name = request.path_params["name"]
    return JSONResponse({"hello": name})


app = Starlette(routes=[Route("/{name}/greeting", homepage)])


from starlette.testclient import TestClient

client = TestClient(app)


def test_redirect_homepage():
    name = "bob%20ross"
    url = f"{name}/greeting"
    for u in [url, url + "/"]:
        resp = client.get(u)
        assert url in str(resp.url)

Expected behavior

Expect that the location header would contain the url to redirect to without modification. If needed, it could replace the actual space character with %20, instead, but it’s not clear to me that is necessary, after looking through the http specs.

This could be accomplished with swapping urllib.quote_plus for urllib.quote in RedirectResponse.

Actual behavior

Spaces are being replaced with the + character.

Environment

• OS: macOS
• Python version: 3.8.8
• Starlette version: 0.13.6 (also separately tested 0.14.2)

[JayH5]

@falkben a pull request would be appreciated

[falkben]

If you’d like I can submit a Pull Request to make the url quoting the same as Django/flask?