Fix TS inconsistency (parse method never returns undefined).

[davidesigner]

In this PR: #179, undefined was added to the type definition and code, then the code was refactored but the type definition was not updated. See the review here: #179 (comment) to understand the history.

[blakeembrey]

Thanks for the PR, but it's correct that decode can return undefined and trying to remove it now would be a breaking change. What you are referring to is the default behavior, which I left designed to mimic 0.x, but the type definition does allow an application to specifically set the decode behavior I wrote in the PR initially in case they wanted to discard values they don't understand.

This is much more relevant when people might want to replace decoding with e.g. base64url.