Enable adaptive-fill-mode

[rasendubi]

It was disabled in 96c8bbf because
haskell-adaptive-fill didn't work well. However, the default adaptive
fill rulles play nicely with Haskell, so it's pretty useful to keep it
enabled even without a custom adaptive-fill-function.

[gracjan]

Can you check what is happening with the tests?

[rasendubi]

Can't say these are real failures. I'd better change tests.

fill-comment-2:

{- a b c d e f g h i j k -}

now fills as

{- a b c d e
 f g h i j
 k -}

instead of

{- a b c d e
f g h i j
k -}

fill-comment-5:

    {-
 a b c d e f g h i
    -}

refills as

    {-
 a b c d e
 f g h i
    -}

instead of

    {-
 a b c d e
f g h i
    -}

This also fixes fill-comment-11 and fill-comment-12:

  --  a
  -- b c d e f g h i j

refills as

  --  a b
  -- c d e
  -- f g h
  -- i j

and

  --  a
  -- b c d e
--f g h
        --            i j

refills as

  --  a b
  -- c d e
  -- f g h
  -- i j

[gracjan]

Good, please change tests, this works much better now.

[rasendubi]

Done.

[gracjan]

Thanks.