Make .Clear() consistent with TagHelperOutput

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

@@ -34,6 +34,7 @@ namespace Microsoft.AspNet.Html.Abstractions
         /// <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 @@ namespace Microsoft.AspNet.Html.Abstractions.Test
                 return this;
             }
 
-            public void Clear()
+            public IHtmlContentBuilder Clear()
             {
                 Entries.Clear();
+                return this;
             }
 
             public void WriteTo(TextWriter writer, IHtmlEncoder encoder)