Fix headers being mutated if passed to web.Response as a CIMultiDict (#10672)

## What do these changes do?

If `CIMultiDict` is passed in we need to make a copy to avoid mutating
it. In some cases we used to copy these twice which was fixed in #10045
but for this case that was the only copy being made and the source of
this regression.

fixes #10670

Author: bdraco

CHANGES/10672.bugfix.rst

@@ -0,0 +1 @@
+Fixed :class:`multidict.CIMultiDict` being mutated when passed to :class:`aiohttp.web.Response` -- by :user:`bdraco`.

aiohttp/web_response.py

@@ -532,10 +532,8 @@ def __init__(
 
         if headers is None:
             real_headers: CIMultiDict[str] = CIMultiDict()
-        elif not isinstance(headers, CIMultiDict):
-            real_headers = CIMultiDict(headers)
         else:
-            real_headers = headers  # = cast('CIMultiDict[str]', headers)
+            real_headers = CIMultiDict(headers)
 
         if content_type is not None and "charset" in content_type:
             raise ValueError("charset must not be in content_type argument")

tests/test_web_response.py

@@ -12,7 +12,7 @@
 
 import aiosignal
 import pytest
-from multidict import CIMultiDict, CIMultiDictProxy
+from multidict import CIMultiDict, CIMultiDictProxy, MultiDict
 
 from aiohttp import HttpVersion, HttpVersion10, HttpVersion11, hdrs, web
 from aiohttp.abc import AbstractStreamWriter
@@ -1384,3 +1384,15 @@ async def test_warn_large_cookie(buf: bytearray, writer: AbstractStreamWriter) -
     assert match is not None
     cookie = match.group(1)
     assert len(cookie) == 4097
+
+
+@pytest.mark.parametrize("loose_header_type", (MultiDict, CIMultiDict, dict))
+async def test_passing_cimultidict_to_web_response_not_mutated(
+    loose_header_type: type,
+) -> None:
+    req = make_request("GET", "/")
+    headers = loose_header_type({})
+    resp = web.Response(body=b"answer", headers=headers)
+    await resp.prepare(req)
+    assert resp.content_length == 6
+    assert not headers