CSV.open: detect BOM by default

Fix GH-301

Reported by Junichi Ito. Thanks!!!

lib/csv.rb

@@ -1581,7 +1581,14 @@ def generate_lines(rows, **options)
     def open(filename, mode="r", **options)
       # wrap a File opened with the remaining +args+ with no newline
       # decorator
-      file_opts = options.dup
+      file_opts = {}
+      have_encoding_options = (options.key?(:encoding) or
+                               options.key?(:external_encoding) or
+                               mode.include?(":"))
+      if not have_encoding_options and Encoding.default_external == Encoding::UTF_8
+        file_opts[:encoding] = "bom|utf-8"
+      end
+      file_opts.merge!(options)
       unless file_opts.key?(:newline)
         file_opts[:universal_newline] ||= false
       end

test/csv/interface/test_read.rb

@@ -215,6 +215,16 @@ def test_open_with_newline
     end
   end
 
+  def test_open_with_bom
+    csv_data = @input.read
+    bom = "\ufeff" # U+FEFF ZERO WIDTH NO-BREAK SPACE
+    File.binwrite(@input.path, "#{bom}#{csv_data}")
+    @input.rewind
+    CSV.open(@input.path, col_sep: "\t") do |csv|
+      assert_equal(@rows, csv.to_a)
+    end
+  end
+
   def test_parse
     assert_equal(@rows,
                  CSV.parse(@data, col_sep: "\t", row_sep: "\r\n"))