let D.E.ASCII conform to the spec

Ignore-this: 788eb99b3948ce201769471919562114

The contract for the "encodeable" function says, in part, "If it yields False,
encodeChar must throw an exception.". This patch makes the ASCII encoding
instance throw an exception for non-ASCII characters.

darcs-hash:20100909222914-76d51-76d9891ae18f13751b004e2b6ed24401192ae79f

Data/Encoding/ASCII.hs

@@ -1,10 +1,12 @@
 {-# LANGUAGE DeriveDataTypeable #-}
 module Data.Encoding.ASCII where
 
+import Control.Throws
 import Data.Char
 import Data.Encoding.Base
 import Data.Encoding.ByteSource
 import Data.Encoding.ByteSink
+import Data.Encoding.Exception
 import Data.Typeable
 
 data ASCII = ASCII deriving (Show,Eq,Typeable)
@@ -13,6 +15,7 @@ instance Encoding ASCII where
     decodeChar _ = do
       w <- fetchWord8
       return $ chr $ fromIntegral w
-    encodeChar _ c = do
-      pushWord8 $ fromIntegral $ ord c
+    encodeChar enc c
+      | encodeable enc c = pushWord8 . fromIntegral . ord $ c
+      | otherwise        = throwException . HasNoRepresentation $ c
     encodeable _ c = c < '\128'