Fix XmlSchemaTest in ReflectionOnly mode. (dotnet/corefx#14596)

* Fix XmlSchemaTest in ReflectionOnly mode.

Fix dotnet/corefx#11918

* Adress feedback.


Commit migrated from https://github.com/dotnet/corefx/commit/5b4c955cf446cc735a45353d46261d5a6823a2fe

src/libraries/System.Private.Xml/src/System/Xml/Serialization/ReflectionXmlSerializationWriter.cs

@@ -387,7 +387,7 @@ private void WriteElement(object o, ElementAccessor element, string arrayName, b
                     throw new PlatformNotSupportedException();
                 }
 
-                WriteStructMethod(mapping, name, ns, o, mapping.TypeDesc.IsNullable, needType: false, parentMapping: parentMapping);
+                WriteStructMethod(mapping, name, ns, o, element.IsNullable, needType: false, parentMapping: parentMapping);
             }
             else if (element.Mapping is SpecialMapping)
             {
@@ -477,7 +477,7 @@ private void WriteStructMethod(StructMapping mapping, string n, string ns, objec
             {
                 if (mapping.TypeDesc.Type != null && typeof(XmlSchemaObject).IsAssignableFrom(mapping.TypeDesc.Type))
                 {
-                    throw new PlatformNotSupportedException(typeof(XmlSchemaObject).ToString());
+                    EscapeName = false;
                 }
 
                 XmlSerializerNamespaces xmlnsSource = null;
@@ -711,12 +711,10 @@ private void WriteMember(object memberValue, AttributeAccessor attribute, TypeDe
                     }
                 }
 
-                var a = memberValue as IEnumerable;
-
-                // #10593: Add More Tests for Serialization Code
-                Debug.Assert(a != null);
-
-                var e = a.GetEnumerator();
+                if (memberValue != null)
+                {
+                    var a = (IEnumerable) memberValue;
+                    IEnumerator e = a.GetEnumerator();
                     bool shouldAppendWhitespace = false;
                     if (e != null)
                     {
@@ -793,6 +791,7 @@ private void WriteMember(object memberValue, AttributeAccessor attribute, TypeDe
                         }
                     }
                 }
+            }
             else
             {
                 WriteAttribute(memberValue, attribute, parent);

src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.cs

@@ -2331,9 +2331,6 @@ public static void XML_TypeWithFieldsOrdered()
         Assert.Equal(value.StringField2, actual.StringField2);
     }
 
-#if ReflectionOnly
-    [ActiveIssue(14260)]
-#endif
     [Fact]
     public static void XmlSchemaTest()
     {