[ruby/csv] Fix a bug that write_nil_value or write_empty_value don't work with non String

GitHub: fix GH-123

Reported by asm256. Thanks!!!

https://github.com/ruby/csv/commit/b4492139be

lib/csv/fields_converter.rb

@@ -50,7 +50,7 @@ class CSV
       fields.collect.with_index do |field, index|
         if field.nil?
           field = @nil_value
-        elsif field.empty?
+        elsif field.is_a?(String) and field.empty?
           field = @empty_value unless @empty_value_is_empty_string
         end
         @converters.each do |converter|

test/csv/write/test_converters.rb

@@ -20,14 +20,14 @@ module TestCSVWriteConverters
   end
 
   def test_nil_value
-    assert_equal(%Q[a,NaN,c\n],
-                 generate_line(["a", nil, "c"],
+    assert_equal(%Q[a,NaN,29\n],
+                 generate_line(["a", nil, 29],
                                write_nil_value: "NaN"))
   end
 
   def test_empty_value
-    assert_equal(%Q[a,,c\n],
-                 generate_line(["a", "", "c"],
+    assert_equal(%Q[a,,29\n],
+                 generate_line(["a", "", 29],
                                write_empty_value: nil))
   end
 end