fix asgi spec violation causing crash in lilya

Detail:

The asgi spec only specifies that the headers are an iterable. Uvicorn
assumes that the headers are a list. This causes a crash at least in
lilya and can cause hard to debug bugs

uvicorn/protocols/http/h11_impl.py

@@ -465,8 +465,11 @@ async def send(self, message: ASGISendEvent) -> None:
 
             status = message["status"]
             headers = self.default_headers + list(message.get("headers", []))
+            # we need to make sure that the headers are a list, the spec requires only an iterator
+            scope_headers = list(self.scope["headers"])
+            self.scope["headers"] = scope_headers
 
-            if CLOSE_HEADER in self.scope["headers"] and CLOSE_HEADER not in headers:
+            if CLOSE_HEADER in scope_headers and CLOSE_HEADER not in headers:
                 headers = headers + [CLOSE_HEADER]
 
             if self.access_log:

uvicorn/protocols/http/httptools_impl.py

@@ -468,8 +468,11 @@ async def send(self, message: ASGISendEvent) -> None:
 
             status_code = message["status"]
             headers = self.default_headers + list(message.get("headers", []))
+            # we need to make sure that the headers are a list, the spec requires only an iterator
+            scope_headers = list(self.scope["headers"])
+            self.scope["headers"] = scope_headers
 
-            if CLOSE_HEADER in self.scope["headers"] and CLOSE_HEADER not in headers:
+            if CLOSE_HEADER in scope_headers and CLOSE_HEADER not in headers:
                 headers = headers + [CLOSE_HEADER]
 
             if self.access_log: