Fix GH-7953: ob_clean() only does not set Content-Encoding

If an output handler has not yet been started, calling `ob_clean()`
causes it to start. If that happens, we must not forget to

Fix GH-7953: ob_clean() only does not set Content-Encoding

If an output handler has not yet been started, calling `ob_clean()`
causes it to start. If that happens, we must not forget to set the
`Content-Encoding` and `Vary` headers.

Closes GH-7960.

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