Deserializer should handle nullable StringEnum<T> (#1760)

* add deserializer tests for nullable enums, StringEnum and nullable StringEnum properties

* Fix deserializing nullable structs by using the underlying type when nullable

* StringEnum<T> should behave like an enum, returning default(T) when it is uninitialised/null/blank

* Don't allow null to be passed into StringEnum ctor - if it needs to be null then it should be declared as nullable

* fix expected json

* move logic to determine if property is a StringEnum<T> into helper function

* serializer needs to treat StringEnum<T> specially by serializing the enum value according to existing serializer strategy (parameter attributes and so on)

* remove fallback to default(T)

* add test to assert ctor throws exception when null passed in

* remove test for default(T) fallback behaviour

* Fix exception in serializer test - StringEnum property must be initialized otherwise an exception is thrown when attempting to serialize

* Dont allow empty strings either

Octokit.Tests/Models/StringEnumTests.cs

@@ -7,6 +7,14 @@ namespace Octokit.Tests.Models
     {
         public class TheCtor
         {
+            [Fact]
+            public void ShouldThrowForNullOrEmptyStringValues()
+            {
+                Assert.Throws<ArgumentNullException>(() => new StringEnum<AccountType>(null));
+
+                Assert.Throws<ArgumentException>(() => new StringEnum<AccountType>(""));
+            }
+
             [Fact]
             public void ShouldSetValue()
             {
@@ -42,16 +50,12 @@ namespace Octokit.Tests.Models
             }
         }
 
-        public class TheParsedValueProperty
+        public class TheValueProperty
         {
-            [Theory]
-            [InlineData("")]
-            [InlineData(null)]
-            [InlineData("Cow")]
-            public void ShouldThrowForInvalidValue(string value)
+            [Fact]
+            public void ShouldThrowForInvalidValue()
             {
-                var stringEnum = new StringEnum<AccountType>(value);
-
+                var stringEnum = new StringEnum<AccountType>("Cow");
                 Assert.Throws<ArgumentException>(() => stringEnum.Value);
             }
 
@@ -96,13 +100,10 @@ namespace Octokit.Tests.Models
                 Assert.True(result);
             }
 
-            [Theory]
-            [InlineData("")]
-            [InlineData(null)]
-            [InlineData("Cow")]
-            public void ShouldReturnFalseForInvalidValue(string value)
+            [Fact]
+            public void ShouldReturnFalseForInvalidValue()
             {
-                var stringEnum = new StringEnum<AccountType>(value);
+                var stringEnum = new StringEnum<AccountType>("Cow");
 
                 AccountType type;
                 var result = stringEnum.TryParse(out type);