Ensure SystemTextJsonHelper always HTML encodes user content

[pranavkm]

As part of https://github.com/dotnet/corefx/issues/37192, JsonSerializerOptions would expose a property that allows customizing the JavaScript encoder. SystemTextJsonHelper uses the user configured JsonSerializerOptions.

We need to do the same thing here as we do with Json.Net i.e. copy the settings, but reset the encoder to ensure it's restrictive.

• https://github.com/aspnet/AspNetCore/blob/master/src/Mvc/Mvc.ViewFeatures/src/Rendering/SystemTextJsonHelper.cs
• https://github.com/aspnet/AspNetCore/blob/master/src/Mvc/Mvc.NewtonsoftJson/src/NewtonsoftJsonHelper.cs#L82-L88

[pranavkm]

cc @ashson