fix failing test case

An empty list of [ContentIdenfier] serialized to the same thing
as a single ContentIdentifier "". Avoid this ambiguity by requiring the
list be non-empty.

Utility/QuickCheck.hs

@@ -17,6 +17,7 @@ import Test.QuickCheck as X
 import Data.Time.Clock.POSIX
 import Data.Ratio
 import System.Posix.Types
+import Data.List.NonEmpty (NonEmpty(..))
 import Prelude
 
 {- Times before the epoch are excluded. Half with decimal and half without. -}
@@ -41,6 +42,10 @@ instance Arbitrary FileID where
 instance Arbitrary FileOffset where
 	arbitrary = nonNegative arbitrarySizedIntegral
 
+{- Quickcheck lacks this instance. -}
+instance Arbitrary l => Arbitrary (NonEmpty l) where
+	arbitrary = (:|) <$> arbitrary <*> arbitrary
+
 nonNegative :: (Num a, Ord a) => Gen a -> Gen a
 nonNegative g = g `suchThat` (>= 0)