ordered_yaml(): omit document header

According to the YAML spec, 'If no explicit header line is specified at the
start of the stream, the processor should behave as if a header line containing
an unadorned “---” was specified.' (<http://yaml.org/spec/1.0/>). Since some
applications barf on the document header, omit it.

Change-Id: Id8163d372669b8d7fcae30e7cd40bbbbf419c7a7