white-listing for export properties

Only properties explicitly inputed into SdfWriter should be
exported. This new behavior could be added with an extra constructor
that takes a Set of properties. Using existing constructors keeps
the old behavior.

Change-Id: I04d42b0d245f5b7479436396efecbcb6daf13669
Unit tests for optional property writing.

Signed-off-by: Egon Willighagen <egonw@users.sourceforge.net>
Removing output from unit tests.

Signed-off-by: Egon Willighagen <egonw@users.sourceforge.net>