Fix moving same level header over code block issue

[syohex]

This is related to #75.
CC: @walmes, @jrblevin

Test code is here.
@walmes Should I add more tests ? If I missed some test cases, please tell me them.

[jrblevin]

This is interesting. There was already a test like this one. In fact, there is a special Markdown file outline-code.text designed to test this. It's strange that those were passing but this new test fails (before your patch).

EDIT: The previous tests are test-markdown-outline/navigation-with-code (line 2675) and test-markdown-outline/visibility-with-code (line 2731).

[jrblevin]

I merged this just now. Thanks once again!