Some Atom feeds not populating Link correctly

[s3ththompson]

Based on the spec, atom:link elements with a rel attribute of alternate or a missing rel attribute should be considered as links.

Currently, line 54 in atom.go correctly sets the link for the latter case, but not the former.

if link.Rel == "" {
  next.Link = link.Href
}

should probably be

if link.Rel == "alternate" || link.Rel == "" {
  next.Link = link.Href
}

This small change fixes a few feeds that were parsing incorrectly for me.

[SlyMarbo]

Thanks for the bug report and the fix.