Check for rel-syndication on a non-post-permalink (e.g. an h-feed)

[karadaisy]

Described here http://indiewebcamp.com/irc/2015-03-06#t1425670273229

rel-syndication is page-scoped, so including them on an h-feed with multiple h-entrys causes problems.

This of course depends on #13 -- tests for h-feed in general