Non-nullable reference types

[luboshl]

• Add support of non-nullable reference types the same way as ASP.NET Core MVC model binding does.
  • Non-nullable reference type behave the same way as it has [Required] attribute - validation fails when the property value is null.
• Fix duplicate validation error messages for the the same field.
• Optimize visiting of enumerable types - exclude string type.
• Add some other complex types to exclude recursion
  • Uri used in tests failed because it has non-nullable reference properties with getter that failed in test cases used in unit tests.
  • Similar exclusion is used in ASP.NET Core - see lines 122-135 in MvcCoreMvcOptionsSetup.cs