Fix parsing values which starts with dash, but has whitespace inside.

this is required when your option can accept command line arguments to other applications.

See dotnet/BenchmarkDotNet#2320 (comment) for example of parsing issues

src/CommandLine/Core/Tokenizer.cs

@@ -28,7 +28,7 @@ public static Result<IEnumerable<Token>, Error> Tokenize(
             Action<Error> onError = errors.Add;
 
             var tokens = (from arg in arguments
-                          from token in !arg.StartsWith("-", StringComparison.Ordinal)
+                          from token in (!arg.StartsWith("-", StringComparison.Ordinal) || arg.Contains(" "))
                                ? new[] { Token.Value(arg) }
                                : arg.StartsWith("--", StringComparison.Ordinal)
                                      ? TokenizeLongName(arg, onError)

tests/CommandLine.Tests/Unit/ParserTests.cs

@@ -163,6 +163,27 @@ public void Parse_options_with_double_dash()
             // Teardown
         }
 
+        [Fact]
+        public void Parse_options_with_whitespace_and_double_dash()
+        {
+            // Fixture setup
+            var expectedOptions = new Simple_Options
+            {
+                StringValue = "--astring value",
+                IntSequence = Enumerable.Empty<int>(),
+            };
+            var sut = new Parser();
+
+            // Exercize system
+            var result =
+                sut.ParseArguments<Simple_Options>(
+                    new[] { "--stringvalue", "--astring value" });
+
+            // Verify outcome
+            ((Parsed<Simple_Options>)result).Value.Should().BeEquivalentTo(expectedOptions);
+            // Teardown
+        }
+
         [Fact]
         public void Parse_options_with_repeated_value_in_values_sequence_and_option()
         {