add 'deprecation' messages for #to_csv arguments

lib/roo/formatters/csv.rb

@@ -1,7 +1,15 @@
 module Roo
   module Formatters
     module CSV
-      def to_csv(filename = nil, sheet = default_sheet, separator: ",")
+      def to_csv(filename = nil, old_separator = nil, old_sheet = nil, separator: ",", sheet: default_sheet)
+        if old_separator
+          warn("[DEPRECATION] optional argument for separator is deprecated. Please use keyword argument :separator instead")
+          separator = old_separator
+        end
+        if old_sheet
+          warn("[DEPRECATION] optional argument for sheet is deprecated. Please use keyword argument :sheet instead")
+          sheet = old_sheet
+        end
         if filename
           File.open(filename, "w") do |file|
             write_csv_content(file, sheet, separator)

spec/lib/roo/base_spec.rb

@@ -271,5 +271,19 @@ def sheets
     it 'should convert the spreadsheet to csv using the separator when is passed on the parameter' do
       expect(spreadsheet.to_csv(separator: ';')).to eq(expected_csv_with_semicolons)
     end
+
+    context 'should contains the deprecation warning message' do
+      it 'convert the spreadsheet to csv using the separator' do
+        converting =-> { spreadsheet.to_csv(nil, ';') }
+        expect(converting.call).to eq(expected_csv_with_semicolons)
+        expect { converting.call }.to output(/DEPRECATION.*:separator\b/).to_stderr
+      end
+
+      it 'be able to arguments: filename, separator, sheet' do
+        converting =-> { spreadsheet.to_csv(nil, ';', spreadsheet.default_sheet) }
+        expect(converting.call).to eq(expected_csv_with_semicolons)
+        expect { converting.call }.to output(/DEPRECATION.*:sheet\b/).to_stderr
+      end
+    end
   end
 end