Fix recognition of no-op change requests for structs

Fix #81

Author: AArnott

src/ImmutableObjectGraph.Generation.Tests/TestSources/ImmutableWithComplexStructField.Tests.cs

@@ -32,7 +32,7 @@ public void StructWithoutOperatorsAlwaysRecreatesObjectWithSameValue()
             Assert.Equal(s1.Field1, v2.SomeStructField.Field1);
         }
 
-        [Fact(Skip = "Not yet passing (#81).")]
+        [Fact]
         public void StructWithoutOperatorsAlwaysRecreatesObjectWithoutValue()
         {
             var s1 = new SomeStructWithMultipleFields(1, 2);
@@ -52,7 +52,7 @@ public void StructWithOperatorsRecreatesObjectWithChangedValue()
             Assert.Equal(s13.Field2, v2.SomeStructFieldWithOperator.Field2);
         }
 
-        [Fact(Skip = "Not yet passing (#81).")]
+        [Fact]
         public void StructWithOperatorsRecyclesObjectWithSameValue()
         {
             var s12 = new SomeStructWithMultipleFieldsAndOperator(1, 2);

src/ImmutableObjectGraph.Generation/CodeGen.cs

@@ -531,7 +531,7 @@ private IEnumerable<MethodDeclarationSyntax> CreateWithCoreMethods()
                             SyntaxFactory.ReturnStatement(
                                 SyntaxFactory.InvocationExpression(
                                     Syntax.ThisDot(WithFactoryMethodName),
-                                    this.CreateArgumentList(this.applyToMetaType.AllFields, ArgSource.OptionalArgumentOrProperty, OptionalStyle.Always)
+                                    this.CreateArgumentList(this.applyToMetaType.AllFields, ArgSource.Argument, OptionalStyle.None)
                                     .AddArguments(SyntaxFactory.Argument(SyntaxFactory.NameColon(IdentityParameterName), NoneToken, Syntax.OptionalFor(Syntax.ThisDot(IdentityPropertyName))))))));
                 }