Handle invalid feeds with relative URLs

reader/atom/atom_03.go

reader/atom/atom_03_test.go

reader/atom/atom_10.go

reader/atom/atom_10_test.go

reader/atom/parser.go

reader/feed/handler.go

reader/json/json.go

reader/json/parser.go

reader/json/parser_test.go

reader/parser/parser.go

reader/parser/parser_test.go

reader/rdf/parser.go

reader/rdf/parser_test.go

reader/rdf/rdf.go

reader/rss/parser.go

reader/rss/parser_test.go

reader/rss/rss.go

url/url_test.go