If AR model provides no "sets", then default should be [] instead of nil to avoid uncaught exception

Previously, if the model defined no sets, and a client asked for them, an uncaught exception would be raised, `NoMethodError: undefined method `detect' for nil`

By making the default return `[]` instead of `nil`, it changes to an appropriate `OAI::NoMatchException` which is rescued properly by controller etc.

I do not use sets, and am not totally sure functionality is working if you do, haven't tracked down a list. But pretty sure this wont' break anything.

Author: jrochkind

lib/oai/provider/model/activerecord_wrapper.rb

@@ -44,7 +44,7 @@ def latest
     # returns all the sets the model supports.  See the
     # activerecord_provider tests for an example.
     def sets
-      model.sets if model.respond_to?(:sets)
+      model.respond_to?(:sets) ? model.sets : []
     end
 
     def find(selector, options={})
@@ -177,7 +177,7 @@ def sql_conditions(opts)
         sql << "#{model.base_class.table_name}.#{timestamp_field} < :until"
         esc_values[:until] = parse_to_local(opts[:until]) { |t| t + 1 }
       end
-      
+
       return [sql.join(" AND "), esc_values]
     end
 
@@ -197,7 +197,7 @@ def parse_to_local(time)
           raise OAI::ArgumentException.new, "unparsable date: '#{time}'"
         end
       end
-        
+
       time_obj = yield(time_obj) if block_given?
 
       if time_obj.kind_of?(Date)

test/activerecord_provider/tc_ar_provider.rb

@@ -40,6 +40,11 @@ def test_list_records_scope
     assert_equal expected_count, doc.elements['OAI-PMH/ListRecords'].to_a.size
   end
 
+  def test_invalid_set_raises_no_match
+    assert_raises(OAI::NoMatchException) do
+      @provider.list_records(:metadata_prefix => 'oai_dc', :set => "invalid_does_not_exist")
+    end
+  end
 
   def test_get_record_alternate_identifier_column
     @provider = ARProviderCustomIdentifierField.new
@@ -127,7 +132,7 @@ def test_from_and_until
       )
     assert_equal 40, doc.elements['OAI-PMH/ListRecords'].to_a.size
   end
-  
+
   def test_bad_until_raises_exception
     DCField.order(id: :asc).limit(10).update_all(updated_at: 1.year.ago)
     DCField.order(id: :desc).limit(10).update_all(updated_at: 1.year.from_now)
@@ -140,11 +145,11 @@ def test_bad_until_raises_exception
       end
     end
   end
-  
+
   def test_bad_from_raises_exception
     DCField.order(id: :asc).limit(10).update_all(updated_at: 1.year.ago)
     DCField.order(id: :desc).limit(10).update_all(updated_at: 1.year.from_now)
-    
+
     badTimes = [
       'junk',
       'February 92nd, 2015']
@@ -169,7 +174,7 @@ def test_handles_empty_collections
       REXML::Document.new(@provider.list_records(:metadata_prefix => 'oai_dc'))
     end
   end
-  
+
   def test_bad_id_raises_exception
     badIdentifiers = [
       'invalid"id',
@@ -183,7 +188,7 @@ def test_bad_id_raises_exception
       end
     end
   end
-  
+
 
   def setup
     @provider = ARProvider.new