Fix rdoc

New rdoc classes/methods have different signatures, and also different behaviour, 
```== FOO```
compiles now to:
```
<h2 id="label-FOO">FOO<span><a href="#label-FOO">&para;</a> <a href="#top">&uarr;</a></span></h2>
```

To keep the old behaviour I rolled-back the rdoc gem to 3.12.x version, and added a regression test.

review @dwradcliffe