Add support for dashdash in unparser for issue #254.

Author: scadorel

Diff for: src/CommandLine/UnParserExtensions.cs

@@ -148,6 +148,12 @@ orderby v.Index
                         .Append(FormatOption((OptionSpecification)opt.Specification, opt.Value, settings))
                         .Append(' ')
                 );
+
+            if (valSpecs.Any() && parser.Settings.EnableDashDash)
+            {
+                builder.Append("-- ");
+            }
+
             valSpecs.ForEach(
                 val => builder.Append(FormatValue(val.Specification, val.Value)).Append(' '));
 

Diff for: tests/CommandLine.Tests/Unit/UnParserExtensionsTests.cs

@@ -65,6 +65,33 @@ public static void UnParsing_instance_with_equal_token_returns_command_line_with
                 .ShouldBeEquivalentTo("-i 1 2 3 --stringvalue=nospaces -x 123456789");
         }
 
+        [Fact]
+        public static void UnParsing_instance_with_dash_in_value_and_dashdash_enabled_returns_command_line_with_value_prefixed_with_dash_dash()
+        {
+            var options = new Simple_Options_With_Values { StringSequence = new List<string> { "-something", "with", "dash" } };
+            new Parser((setting) => setting.EnableDashDash = true)
+                .FormatCommandLine(options)
+                .ShouldBeEquivalentTo("-- -something with dash");
+        }
+
+        [Fact]
+        public static void UnParsing_instance_with_no_values_and_dashdash_enabled_returns_command_line_without_dash_dash()
+        {
+            var options = new Simple_Options_With_Values();
+            new Parser((setting) => setting.EnableDashDash = true)
+                .FormatCommandLine(options)
+                .ShouldBeEquivalentTo("");
+        }
+
+        [Fact]
+        public static void UnParsing_instance_with_dash_in_value_and_dashdash_disabled_returns_command_line_with_value()
+        {
+            var options = new Simple_Options_With_Values { StringSequence = new List<string> { "-something", "with", "dash" } };
+            new Parser()
+                .FormatCommandLine(options)
+                .ShouldBeEquivalentTo("-something with dash");
+        }
+
         public static IEnumerable<object> UnParseData
         {
             get