test that oai_dc metadata prefix is always requested explicitly

Author: eilmiv

test/unit/ingestors/oai_pmh_test.rb

@@ -25,7 +25,7 @@ def initialize(rdf_strings, dc_strings)
   def list_records(metadata_prefix: nil)
     if metadata_prefix == 'rdf'
       @rdf_response
-    else
+    elsif metadata_prefix == 'oai_dc'
       @dc_response
     end
   end