Don't group duplicated headers on a single string on the TestClient

[ooknimm]

Summary

The testclient handles headers with duplicate field names differently.

when request with headers with duplicate field name, starlette can handles like this

starlette

# request header
[("x-token", "foo"), ("x-token", "bar")]

# in starlette
request.headers.getlist("x-token")
--> ["foo", "bar"]

but testclient(httpx client) not handles like that

testclient

# request header
[("x-token", "foo"), ("x-token", "bar")]

# in starlette
request.headers.getlist("x-token")
--> ["foo, bar"]

so i fixed the testclient same as starlette to handle headers with duplicate field name

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[ooknimm]

headers.raw includes upper case key

i found multi_items method that lower case key.

[Kludex]

Thanks @ooknimm 🙏