Exactprint: HsLet should needs a leading space

In Wingman, running "Complete case constructors" on the following:

```haskell
let occs = case params of
```

produces the rather ugly:

```haskell
let occs =case params of
IntroduceAllUnnamed -> _
IntroduceOnlyNamed ons -> _
IntroduceOnlyUnnamed n -> _
```

note the missing space before `case`!

I'd think this is as easy as changing this:

https://github.com/haskell/haskell-language-server/blob/master/ghcide/src/Development/IDE/GHC/ExactPrint.hs#L203

but it looks like it's already supposed to insert a space! Is there some other node in the tree that's getting in the way?