Add compresslevel to GZipMiddleware

Current default level = 9
New default level = 1

Documentation gzip: https://docs.python.org/3/library/gzip.html
Benchmarks: https://tukaani.org/lzma/benchmarks.html

Reformate code

starlette/middleware/gzip.py

@@ -6,29 +6,36 @@
 
 
 class GZipMiddleware:
-    def __init__(self, app: ASGIApp, minimum_size: int = 500) -> None:
+    def __init__(
+        self, app: ASGIApp, minimum_size: int = 500, compresslevel: int = 1
+    ) -> None:
         self.app = app
         self.minimum_size = minimum_size
+        self.compresslevel = compresslevel
 
     async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
         if scope["type"] == "http":
             headers = Headers(scope=scope)
             if "gzip" in headers.get("Accept-Encoding", ""):
-                responder = GZipResponder(self.app, self.minimum_size)
+                responder = GZipResponder(
+                    self.app, self.minimum_size, compresslevel=self.compresslevel
+                )
                 await responder(scope, receive, send)
                 return
         await self.app(scope, receive, send)
 
 
 class GZipResponder:
-    def __init__(self, app: ASGIApp, minimum_size: int) -> None:
+    def __init__(self, app: ASGIApp, minimum_size: int, compresslevel: int = 1) -> None:
         self.app = app
         self.minimum_size = minimum_size
         self.send = unattached_send  # type: Send
         self.initial_message = {}  # type: Message
         self.started = False
         self.gzip_buffer = io.BytesIO()
-        self.gzip_file = gzip.GzipFile(mode="wb", fileobj=self.gzip_buffer)
+        self.gzip_file = gzip.GzipFile(
+            mode="wb", fileobj=self.gzip_buffer, compresslevel=compresslevel
+        )
 
     async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
         self.send = send