Fixed failed parsing on nullable options

Added unit test for nullable types

Author: Adam Byrd

src/CommandLine.Tests/CommandLine.Tests.csproj

@@ -58,6 +58,7 @@
     <Compile Include="..\SharedAssemblyInfo.cs">
       <Link>Properties\SharedAssemblyInfo.cs</Link>
     </Compile>
+    <Compile Include="Fakes\FakeOptionsWithNullable.cs" />
     <Compile Include="Fakes\FakeOptionsWithTwoIntegers.cs" />
     <Compile Include="Fakes\FakeInterfaceOptions.cs" />
     <Compile Include="Fakes\FakeOptionsWithHelpTextEnum.cs" />

src/CommandLine.Tests/Fakes/FakeOptionsWithNullable.cs

@@ -0,0 +1,11 @@
+namespace CommandLine.Tests.Fakes
+{
+    class FakeOptionsWithNullable
+    {
+        [Option('n')]
+        public int? NullableIntValue { get; set; }
+
+        [Option('c')]
+        public Colors? NullableColorsValue { get; set; }
+    }
+}

src/CommandLine.Tests/Unit/ParserTests.cs

@@ -227,5 +227,25 @@ public void Parse_verbs_using_generic_overload()
             Assert.False(result.Errors.Any());
             // Teardown
         }
+
+        [Fact]
+        public void Parse_nullable_options()
+        {
+            // Fixture setup
+            var expectedOptions = new FakeOptionsWithNullable
+            {
+                NullableIntValue = 60,
+                NullableColorsValue = Colors.Red
+            };
+            var sut = new Parser();
+
+            // Exercize system
+            var result = sut.ParseArguments<FakeOptionsWithNullable>(new[] { "-n", "60", "-c", "Red" });
+
+            // Verify outcome
+            result.Value.ShouldHave().AllProperties().EqualTo(expectedOptions);
+            Assert.False(result.Errors.Any());
+            // Teardown
+        }
     }
 }

src/CommandLine/Core/TypeConverter.cs

@@ -41,6 +41,8 @@ private static Maybe<object> ChangeType(string value, Type conversionType, Cultu
         {
             try
             {
+                conversionType = Nullable.GetUnderlyingType(conversionType) ?? conversionType;
+
                 return Maybe.Just(
                     MatchBoolString(value)
                         ? ConvertBoolString(value)