fix(logs): declare the compressed length so a log download finishes

The gzip middleware compressed a text body but kept the handler's declared
Content-Length (the uncompressed size), so a browser waited for bytes that
never came: downloading any log over 1 KB hung, in the workspace and on the
admin page. The middleware now drops a declared length before compressing.

Part of #329

py/api/__init__.py

@@ -295,6 +295,10 @@ async def _gzip_middleware(request, handler):
     if len(compressed) >= len(body):
         return response
 
+    # A length the handler declared is the uncompressed one: drop it so the
+    # compressed body's own length is sent (#329: a stale length made the
+    # browser wait for bytes that never came).
+    response.headers.pop("Content-Length", None)
     response.body = compressed
     response.headers["Content-Encoding"] = "gzip"
     response.headers["Vary"] = "Accept-Encoding"