Boy Scout: Create document parser spec file

Regroup DocumentParser spec. Now they're duplicates in
local_census_specs and census_api_spec.

Move duplicate private method "dni?(document_type) from
LocalCensus and CensusAPI to DocumentParser

lib/document_parser.rb

@@ -57,4 +57,9 @@ module DocumentParser
     end
     variants
   end
+
+  def dni?(document_type)
+    document_type.to_s == "1"
+  end
+
 end