Rename two tag REST test classes (rename-only commit).

--HG--
rename : modules/tag/tests/Tag_Item_Rest_Test.php => modules/tag/tests/Rest_TagItems_Test.php
rename : modules/tag/tests/Tags_Rest_Test.php => modules/tag/tests/Rest_Tags_Test.php
Fix class names for renamed tests, and move Tag_Rest tests into Rest_Tags_Test.
Fix bug with REST relationship PUT/POST members.
Rewrite tag-related REST unit tests.
Merge pull request #425 from shadlaws/fixes_20130620

Fixes 20130620