Add support for transcoding String CSV

GitHub: fix #254

Syntax is "from-encoding:to-encoding".

Reported by Richard Stueven. Thanks!!!

lib/csv.rb

@@ -1889,8 +1889,19 @@ def initialize(data,
     raise ArgumentError.new("Cannot parse nil as CSV") if data.nil?
 
     if data.is_a?(String)
+      if encoding
+        if encoding.is_a?(String)
+          data_external_encoding, data_internal_encoding = encoding.split(":", 2)
+          if data_internal_encoding
+            data = data.encode(data_internal_encoding, data_external_encoding)
+          else
+            data = data.dup.force_encoding(data_external_encoding)
+          end
+        else
+          data = data.dup.force_encoding(encoding)
+        end
+      end
       @io = StringIO.new(data)
-      @io.set_encoding(encoding || data.encoding)
     else
       @io = data
     end

test/csv/test_encodings.rb

@@ -288,6 +288,37 @@ def test_invalid_encoding_row_error
                  error.message)
   end
 
+  def test_string_input_transcode
+    # U+3042 HIRAGANA LETTER A
+    # U+3044 HIRAGANA LETTER I
+    # U+3046 HIRAGANA LETTER U
+    value = "\u3042\u3044\u3046"
+    csv = CSV.new(value, encoding: "UTF-8:EUC-JP")
+    assert_equal([[value.encode("EUC-JP")]],
+                 csv.read)
+  end
+
+  def test_string_input_set_encoding_string
+    # U+3042 HIRAGANA LETTER A
+    # U+3044 HIRAGANA LETTER I
+    # U+3046 HIRAGANA LETTER U
+    value = "\u3042\u3044\u3046".encode("EUC-JP")
+    csv = CSV.new(value.dup.force_encoding("UTF-8"), encoding: "EUC-JP")
+    assert_equal([[value.encode("EUC-JP")]],
+                 csv.read)
+  end
+
+  def test_string_input_set_encoding_encoding
+    # U+3042 HIRAGANA LETTER A
+    # U+3044 HIRAGANA LETTER I
+    # U+3046 HIRAGANA LETTER U
+    value = "\u3042\u3044\u3046".encode("EUC-JP")
+    csv = CSV.new(value.dup.force_encoding("UTF-8"),
+                  encoding: Encoding.find("EUC-JP"))
+    assert_equal([[value.encode("EUC-JP")]],
+                 csv.read)
+  end
+
   private
 
   def assert_parses(fields, encoding, **options)