go fmt
Make OPML v2 header elements optional

Empty, optional header fields should not generate output.
Timestamp format based on RFC 822

The OPML v2 spec says that timestamps should be encoded according to RFC
822.  This format is one variation on that RFC which matches the
examples given at
http://hosting.opml.org/dave/spec/subscriptionList.opml
Use OpmlOutline2 for OpmlBody2