InputTagHelper doesn't copy pre-existing attributes correctly for checkboxes.

[NTaylorMullen]

Given the following TagHelper:

[HtmlTargetElement("input", Attributes = "asp-for,[type=checkbox]")]
public class FooTagHelper : TagHelper
{
    public override int Order => -2000;

    public override void Process(TagHelperContext context, TagHelperOutput output)
    {
        var tb = new TagBuilder("input");
        tb.AddCssClass("myclass");
        output.MergeAttributes(tb);
    }
}

And the following source:

<input asp-for="BoolVal" class="my-control" type="checkbox" />

You end up getting an invalid class attribute value:

<input checked="checked" class="Microsoft.AspNetCore.Mvc.TagHelpers.TagHelperOutputExtensions&#x2B;ClassAttributeHtmlContent" data-val="true" data-val-required="The BoolVal field is required." id="BoolVal" name="BoolVal" type="checkbox" value="true" /><input name="BoolVal" type="hidden" value="false" />

This is because of how we copy the pre-existing output attributes.

[dougbu]

Root cause is the HtmlGenerator does not handle IHtmlContent in htmlAttribute dictionaries it's given. No need for such support elsewhere; the MVC tag helpers should be passing htmlAttributes: null except in this one case.

This problem would mostly go away if we fixed #5902. But, might still need to do something special when !ViewContext.FormContext.CanRenderAtEndOfForm.