fix elapsed to always use floating point notation

unwind/middleware/responsetime.py

@@ -20,7 +20,7 @@ class ResponseTimeMiddleware:
             if message["type"] == "http.response.start":
                 headers = MutableHeaders(scope=message)
                 elapsed = perf_counter() - start
-                headers.append(self.header_name, str(elapsed))
+                headers.append(self.header_name, f"{elapsed:.20f}")
             await send(message)
 
         await self.app(scope, receive, send_wrapper)