Fix FormTagHelper to make redudant Antiforgery token registration a no-op

[sebastienros]

In a form generated from a tag helper, adding @Html.AntiforgeryToken() will result in this cryptic error that doesn't help:

An unhandled exception occurred while processing the request.

FormatException: The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters.
System.Convert.FromBase64_Decode(Char* startInputPtr, Int32 inputLength, Byte* startDestPtr, Int32 destLength)

InvalidOperationException: The antiforgery token could not be decrypted.
Microsoft.AspNetCore.Antiforgery.Internal.DefaultAntiforgeryTokenSerializer.Deserialize(String serializedToken)

[rynowak]

That just sounds like a bug, this shouldn't throw

[dougbu]

@sebastienros is your repro as simple as the following?

<form asp-action="Index">
  @Html.AntiforgeryToken()
</form>

If not, what else is needed?

Head-scratcher: The Html.AntiforgeryToken() call should be redundant.

[sebastienros]

Exactly what I did.

[rynowak]

@kichalla Can you rename this issue so that it shows up better in release notes

[kichalla]

Sure, done.