Revert removed `download` for `DioMixin` #523

Job	Run time
Check formatting (dio)	13s
Check formatting (example)	12s
Check formatting (example_flutter_app)	13s
Check formatting (plugins/cookie_manager)	16s
Check formatting (plugins/http2_adapter)	12s
Check formatting (plugins/native_dio_adapter)	12s
Analyze Flutter libraries (example_flutter_app)	35s
Analyze Flutter libraries (plugins/native_dio_adapter)	38s
Analyze Dart-only libraries (dio)	27s
Analyze Dart-only libraries (example)	23s
Analyze Dart-only libraries (plugins/cookie_manager)	20s
Analyze Dart-only libraries (plugins/http2_adapter)	19s
Run unit tests with dio	0s
Run unit tests with Flutter plugins	0s
Run unit tests with plugins	0s
	4m 0s

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