Fix failed to serialize type info in Xamdroid because it returns "neutral" instead of null for culture-neutral assemblies.

Author: yfakariya

src/MsgPack/Serialization/Polymorphic/TypeInfoEncoder.cs

@@ -67,7 +67,11 @@ public static void Encode( Packer packer, Type type )
 				.Pack( compressedTypeName )
 				.Pack( assemblyName.Name )
 				.Pack( version )
+#if !XAMIOS && !XAMDROID
 				.Pack( assemblyName.GetCultureName() )
+#else
+				.Pack( assemblyName.GetCultureName() == "neutral" ? null : assemblyName.GetCultureName() )
+#endif // !XAMIOS && !XAMDROID
 				.Pack( assemblyName.GetPublicKeyToken() );
 		}