Fixed taxon_preview helper to work properly with postgresql ordering. Fixes #4382
The order is a default scope so doesnt need to be set here just the select distinct change.
Use stub_model rathen than mock_model

Make rails 4.0.4 build happy
Save Asset rather than touch

spree_assets doesnt have timestamps fields and as of rails 4.0.4 it wont
touch associations anymore for models that dont have timestamps fields