The behaviour of `WriteSafeString` changes depending on when my helper is registered

[markdebhailis]

Describe the bug

The behaviour of WriteSafeString changes depending on when my helper is registered

Expected behavior:

A clear and concise description of what you expected to happen.

Test to reproduce

public class WriteSafeStringTest
{
    [Fact]
    public void ItAllowsLateBindingOfHelpers()
    {
        // Arrange
        HandlebarsHelper link_to = (writer, context, parameters) =>
        {
            writer.WriteSafeString($"<a href='{context["url"]}'>{context["text"]}</a>");
        };

        string source = @"Click here: {{link_to}}";

        var data = new
        {
            url = "https://github.com/rexm/handlebars.net",
            text = "Handlebars.Net"
        };

        // Act
        var handlebars1 = HandlebarsDotNet.Handlebars.Create();
        handlebars1.RegisterHelper("link_to", link_to);
        var template1 = handlebars1.Compile(source);
        var result1 = template1(data);

        var handlebars2 = HandlebarsDotNet.Handlebars.Create();
        var template2 = handlebars2.Compile(source);
        handlebars2.RegisterHelper("link_to", link_to);
        var result2 = template2(data);

        // Assert
        Assert.Equal(result1, result2);
    }
}

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