streamDecodeUtf8With: accumulate undecoded chunks correctly

We had previously gotten the accounting and reporting wrong if an
incomplete input was fed in over the course of several continuations,
such that we'd report only the incomplete input seen by the most
recent continuation.

This fixes gh-70.

Data/Text/Encoding.hs

@@ -238,12 +238,13 @@ streamDecodeUtf8 = streamDecodeUtf8With strictDecode
 -- | Decode, in a stream oriented way, a 'ByteString' containing UTF-8
 -- encoded text.
 streamDecodeUtf8With :: OnDecodeError -> ByteString -> Decoding
-streamDecodeUtf8With onErr = decodeChunk 0 0
+streamDecodeUtf8With onErr = decodeChunk B.empty 0 0
  where
   -- We create a slightly larger than necessary buffer to accommodate a
   -- potential surrogate pair started in the last buffer
-  decodeChunk :: CodePoint -> DecoderState -> ByteString -> Decoding
-  decodeChunk codepoint0 state0 bs@(PS fp off len) =
+  decodeChunk :: ByteString -> CodePoint -> DecoderState -> ByteString
+              -> Decoding
+  decodeChunk undecoded0 codepoint0 state0 bs@(PS fp off len) =
     runST $ (unsafeIOToST . decodeChunkToBuffer) =<< A.new (len+1)
    where
     decodeChunkToBuffer :: A.MArray s -> IO Decoding
@@ -281,8 +282,11 @@ streamDecodeUtf8With onErr = decodeChunk 0 0
                       return $! textP arr 0 (fromIntegral n)
                   lastPtr <- peek curPtrPtr
                   let left = lastPtr `minusPtr` curPtr
-                  return $ Some chunkText (B.drop left bs)
-                           (decodeChunk codepoint state)
+                      undecoded = case state of
+                        UTF8_ACCEPT -> B.empty
+                        _           -> B.append undecoded0 (B.drop left bs)
+                  return $ Some chunkText undecoded
+                           (decodeChunk undecoded codepoint state)
         in loop (ptr `plusPtr` off)
   desc = "Data.Text.Internal.Encoding.streamDecodeUtf8With: Invalid UTF-8 stream"