Differing AST on empty Haddock section headings

[amesgen]

Describe the bug

The snippet

{- ***
   aaa
-}

is formatted to

-- ***
--   aaa

which has differing AST. In the first snippet, the parser detects a Haddock section heading, but not in the second one.

Expected behavior
Two options:

• It should not fail with a "differing AST" error, but with the same output.
• Alternatively, this would probably also be solved if we would preserve the multiline comment style of Haddock comments, see Multiline Haddock comments are "normalized" into single-line Haddock comments #641.

Environment
0.3.1.0 and latest master

Additional context
Found via #821 as a side effect, as constructs like this often appear as big comments (definitely not intended as Haddock markup):

{- *********************************************************************
*                                                                      *
                     Id keys
*                                                                      *
********************************************************************* -}