Nested indentation fix for nested list items with adjacent parent list items

Problem: given a nested list between adjacent list items like this:

````
<ul>
  <li>alpha</li>
  <li>bravo
    <ul>
      <li>bravo alpha</li>
      <li>bravo bravo
    </ul>
  </li>
  <li>charlie</li>
</ul>
````

Reverse markdown produces an extra newline:

````
- alpha
- bravo
  - bravo alpha
  - bravo bravo

- charlie
````

This PR fixes the `li` converter and removes the extra newline to produce this markdown:

````
- alpha
- bravo
  - bravo alpha
  - bravo bravo
- charlie
````

Specs included.