StringHtmlContent omitting characters when encoding

Describe the bug

When a StringHtmlContent writes a string containing a surrogate pair, it omits the immediate following single non-supplementary character.

To Reproduce

Steps to reproduce the behavior:

1. Using this version of ASP.NET Core 2.2.5
2. Run this code

    var tearsOfJoy2 = new StringHtmlContent("😂2");
    using (var stringWriter = new StringWriter())
    {
        tearsOfJoy2.WriteTo((TextWriter) stringWriter, HtmlEncoder.Default);
        var encoded = stringWriter.ToString();
    }

Expected behaviour

encoded == "😂2"

Actual behaviour

encoded == "😂"

Additional context

I have a feeling that the issue is within System.Text.Encodings.Web.TextEncoder. If the the emoji is followed by 2 characters, like new StringHtmlContent("😂22"), it seems to be working correctly.

[AbhiAgarwal192]

Submitted a PR for this. This will get fixed in the next release.

[AbhiAgarwal192]

@ryanbrandenburg I have found the actual issue. Will fix it.