allow the HOEDOWN_EXT_UNDERLINE to work

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.

1 files changed, 5 insertions, 0 deletions

diff --git a/test/config.json b/test/config.json
index b6ecb55..d3e170e 100644
--- a/test/config.json
+++ b/test/config.json
@@ -101,6 +101,11 @@
             "input": "Tests/Math.text",
             "output": "Tests/Math.html",
             "flags": ["--math"]
+        },
+        {
+            "input": "Tests/Underline.text",
+            "output": "Tests/Underline.html",
+            "flags": ["--underline"]
         }
     ]
 }