Do not repeat status line in defaultHandler

Instead of repeating the status line in the defaultHandler for every
exception, repeat it only when the message is empty.

Signed-off-by: Jan Hamal Dvořák <mordae@anilinux.org>

lib/Hikaru/Types.hs

@@ -312,7 +312,7 @@ where
 
     where
       response st = resp . responseLBS st [(hContentType, "text/plain")]
-                         . cs . (<> "\n" <> msg)
+                         . cs . (\str -> if msg == "" then str else msg)
 
 
   -- |