fix parser issue with input ending with non-space characters

[d5]

Discovered that the input string that ends with non-whitespace characters caused an incorrect parsing. For example, the following input (fenced code block with no newlines at the end)

```
abc
```

is parsed as a fenced code block followed by another paragraph with text value of "`".

I don't know if the spec requires all input string to end with 1+ newline characters or not, but, appending a whitespace character at the end fixed most of my own use cases. Feel free to close/ignore if you have a better fix 😄 .

Thanks!

[yuin]

Thanks for reporting this, but this PR contains some problems. So I will try to fix the issue by myself.