quickcheck-text generates Text values containing invalid Unicode characters

[williamyaoh]

Specifically, U+FFFE and U+FFFF, which are invalid.

To replicate:

λ> :seti -XTypeApplications
λ> import Data.Text as T
λ> :m + Data.Text.Arbitrary Test.QuickCheck
λ> invalid1 <- generate $ arbitrary @Text `suchThat` (not . T.null . T.filter (== '\65535'))
λ> invalid2 <- generate $ arbitrary @Text `suchThat` (not . T.null . T.filter (== '\65534'))