GFilterOutputStream, from which CamelFilterOutputStream is derived,
defaults to closing its base stream when the instance is finalized.

That makes sense in the general case, but CamelFilterOutputStreams are
usually temporary wrappers on a more permanent base stream, so we need
to remember to override the default "close-base-stream" property value.

I was tempted to just make CamelFilterOutputStream override the default
automatically, but I decided against it because my long term plan is to
drop CamelFilterOutputStream for GConverterOutputStream which also does
not override the "close-base-stream" default.  The closer the semantics
of the two classes match, the easier porting will be in the future.