Only remove space in AutocorrectAlignment#autocorrect

There was a bug that would remove other things than space if one of
the lines in a node with too much indentation was empty or very short.

Also, adding indentation would add trailing whitespace on emtpy lines.
Only auto-correct the first expression of a group in IndentationWidth

It's the job of IndentationConsistency to make sure that all the
statements of a def, for example, are lined up.
Merge pull request #845 from jonas054/fix_autocorrect

Fix two bugs in autocorrect