DefaultSerializer may fail when it encounters Unicode surrogate pairs on buffer boundaries

see https://www.couchbase.com/forums/t/problem-deserializing-documents-with-certain-characters-in-specific-positions/37299

Exception: The output char buffer is too small to contain the decoded characters, encoding ‘Unicode (UTF-8)’ fallback ‘System.Text.DecoderReplacementFallback’.

This is unlikely to occur as surrogate pairs in strings are rare and the buffer boundary is every 1023 characters. But it does affect all framework versions, not just .NET 4. This appears to have been introduced by NCBC-3421