Make .Clear() consistent with TagHelperOutput

Author: rynowak

src/Microsoft.AspNet.Html.Abstractions/IHtmlContentBuilder.cs

@@ -34,6 +34,7 @@ public interface IHtmlContentBuilder : IHtmlContent
  /// <summary>
  /// Clears the content.
  /// </summary>
- void Clear();
+ /// <returns>The <see cref="IHtmlContentBuilder"/>.</returns>
+ IHtmlContentBuilder Clear();
  }
 }

test/Microsoft.AspNet.Html.Abstractions.Test/HtmlContentBuilderExtensionsTest.cs

@@ -373,9 +373,10 @@ public IHtmlContentBuilder AppendEncoded(string encoded)
  return this;
  }
 
- public void Clear()
+ public IHtmlContentBuilder Clear()
  {
  Entries.Clear();
+ return this;
  }
 
  public void WriteTo(TextWriter writer, IHtmlEncoder encoder)