When the `HOEDOWN_EXT_UNDERLINE` extension was enabled, underlined spans
would actually be passed verbatim to the output buffer. This was because
the active_char was _only_ set when the emphasis, double_emphasis, or
triple_emphasis handlers were registered. As a result, no active char
was found in the input buffer, so everything was passed through
verbatim.
This patch fixes this by also registering the `active_char` if the
underline handler is registered. I also added a simple regression test.
I personally don't use this extension, but I encountered this bug over
the course of writing bindings for Rust.
