Optimize WebUtility.HtmlEncode and WebUtility.HtmlDecode

[safern]

When porting WebUtility missing members:

    public static class WebUtility {
        public static void HtmlDecode(string value, TextWriter output);
        public static void HtmlEncode(string value, TextWriter output);
    }

We found that we already had a private method on .NET Core that would do exactly the same as this two members but instead of having a TextWriter as the output they have a StringBuilder. This members where ported on PR dotnet/corefx#13875 with a simple solution of just calling the existing private member and writing the result to the TextWriter output Here is the source code.

On PR dotnet/corefx#13875 we had a large discussion with different solutions. With a conclusion that the best one is @jkotas's solution.

cc @danmosemsft @AlexGhiondea

[karelz]

@benjamin-hodgson thanks for working on the issue!
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