"Formatting should be idempotent" test failure with ASCII style and HTTP headers

```
$ cabal test dhall:tasty --test-options "--quickcheck-tests 1000000 -p idempo"
...
Formatting should be idempotent: FAIL (9.77s)
*** Failed! Falsified (after 40534 tests and 9 shrinks):
ASCII
Header ""
Embed (Import {importHashed = ImportHashed {hash = Nothing, importType = Remote (URL {scheme = HTTP, authority = "a", path = File {directory = Directory {components = []}, file = ""}, query = Nothing, headers = Just (Pi "_" (Var (V "" 0)) (Var (V "" 0)))})}, importMode = Code})
"http://a/ using `` \8594 ``\n" /= "http://a/ using `` -> ``\n"
Use --quickcheck-replay=23582 to reproduce.
```