TagHelperContent.AppendHtml does not append but replace

### Describe the bug
In a TagHelper, I use the AppendHtml method to append `` elements to a `` element. This works if I don't already define options in the select HTML code. But if there's already options in there, the tag helper just deletes them and only the options added by the tag helper remain. The predefined options are gone. That's not what I understand as "append".

### To Reproduce
HTML:
```

(keine)

```

C#:
```
[HtmlTargetElement("select")]
public class SelectTagHelper : TagHelper
{
private readonly IHtmlGenerator generator;

public SelectTagHelper(IHtmlGenerator generator)
{
this.generator = generator;
}

[HtmlAttributeNotBound]
[ViewContext]
public ViewContext ViewContext { get; set; }

[HtmlAttributeName("asp-for")]
public ModelExpression For { get; set; }

public IEnumerable Options { get; set; }

public override void Init(TagHelperContext context)
{
if (For == null)
{
context.Items[typeof(SelectTagHelper)] = null;
return;
}

var realModelType = For.ModelExplorer.ModelType;
allowMultiple = realModelType != typeof(string) && typeof(IEnumerable).IsAssignableFrom(realModelType);
currentValues = generator.GetCurrentValues(ViewContext, For.ModelExplorer, For.Name, allowMultiple);

context.Items[typeof(SelectTagHelper)] = currentValues;
}

public override void Process(TagHelperContext context, TagHelperOutput output)
{
if (Options != null)
{
var listItemBuilder = new HtmlContentBuilder(Options.Count());

foreach (var option in Options)
{
var tagBuilder = new TagBuilder("option");
tagBuilder.InnerHtml.SetContent(option.Text);
tagBuilder.Attributes["value"] = option.Value;
if (option.Summary != null)
tagBuilder.Attributes["data-summary"] = option.Summary;
if (option.HtmlText != null)
tagBuilder.Attributes["data-html"] = option.HtmlText;
if (option.HtmlSummary != null)
tagBuilder.Attributes["data-summary-html"] = option.HtmlSummary;
if (option.Selected || currentValues?.Contains(option.Value) == true)
tagBuilder.Attributes["selected"] = "selected";
if (option.Disabled)
tagBuilder.Attributes["disabled"] = "disabled";

listItemBuilder.AppendLine(tagBuilder);
}

// The following line removes already existing options, see HTML above
output.Content.AppendHtml(listItemBuilder);
}
}
}
```

### Further technical details
- ASP.NET Core version: 3.1.11
- Include the output of `dotnet --info`

```
.NET SDK (gemäß "global.json"):
Version: 5.0.102
Commit: 71365b4d42

Laufzeitumgebung:
OS Name: Windows
OS Version: 10.0.19041
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\5.0.102\

Host (useful for support):
Version: 5.0.2
Commit: cb5f173b96

.NET SDKs installed:
5.0.102 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.All 2.1.24 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.24 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.24 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.11 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.2 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
```

- The IDE (VS / VS Code/ VS4Mac) you're running on, and its version: Visual Studio 2019 16.8.4, Windows 10 x64