fix: TLS backends return case-insensitive response headers

CurlCffi.raw_request and Noble's _flatten_headers built Response.headers as a plain
case-sensitive dict, while aioweb's aiohttp-backed path returns a CIMultiDict -
resp.headers.get('content-type') silently returned None on TLS backends when the
server sent 'Content-Type', contradicting the "backends behave identically" claim.
Both paths now build a multidict.CIMultiDict instead.

Signed-off-by: disqualifier <dev@disqualifier.me>

src/aioweb_tls/backends.py

@@ -9,6 +9,7 @@ import logging
 import math
 
 import aiohttp
+from multidict import CIMultiDict
 from aioweb import Response
 
 log = logging.getLogger(__name__)
@@ -77,13 +78,14 @@ def _noble_content(response) -> bytes:
     return text.encode()
 
 
-def _flatten_headers(headers) -> dict:
+def _flatten_headers(headers) -> CIMultiDict:
     """flatten a Go-style map[str][]str header dict (multi-valued headers as a list)
-    into plain str values, joined with ", " per RFC 7230"""
-    return {
-        key: ", ".join(value) if isinstance(value, list) else value
+    into plain str values, joined with ", " per RFC 7230, into a case-insensitive
+    mapping matching aioweb's aiohttp-backed Response.headers"""
+    return CIMultiDict(
+        (key, ", ".join(value) if isinstance(value, list) else value)
         for key, value in headers.items()
-    }
+    )
 
 
 def _history_entries(history) -> list:
@@ -177,7 +179,7 @@ class CurlCffi:
         content = response.content if response.content is not None else b""
         return Response(
             status_code=response.status_code,
-            headers=dict(response.headers),
+            headers=CIMultiDict(response.headers),
             content=content,
             url=str(response.url),
             reason=getattr(response, "reason", None),