streamDecodeUtf8With: accumulate undecoded chunks correctly
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.

text-utf8/text,beni55/text,hvr/text,text-utf8/text,23Skidoo/text,ekmett/text,bgamari/text,bgamari/text,bgamari/text,bos/text,aloiscochard/text,bgamari/text,aloiscochard/text,spencerjanssen/text,hvr/text,ekmett/text,spencerjanssen/text,ekmett/text,hvr/text,23Skidoo/text,text-utf8/text,23Skidoo/text,text-utf8/text,spencerjanssen/text,aloiscochard/text,beni55/text,alexliew/text,hvr/text,23Skidoo/text,ekmett/text,bos/text,aloiscochard/text,alexliew/text,beni55/text,alexliew/text,spencerjanssen/text,beni55/text,bos/text,alexliew/text,bos/text