When the Content-Type header is processed by ContentExchange::onResponseHeader, it doesn't reconise it. Therefore, the charset part of the Content-Type is not read and the default encoding is not changed in attribute _encoding. As a result, any response in a charset that is not utf-8 gets incorrectly handled when using getResponseContent().
Some test shown that ContentExchange line 114, instead of
int header = HttpHeaders.CACHE.getOrdinal(value);
int header = HttpHeaders.CACHE.getOrdinal(name);