Fix two problems with chunked gzipped HTTP replies.

* receiveEnd is set in a different place in case of chunked transfers,
which would cause the decompressor to never be flushed.
* In the case of chunked transfers, we call Flush() without any input
data (to flush only whatever is remaining in the decompression buffer).
This causes ZLib to return Z_BUF_ERROR which is translated to
B_BUFFER_OVERFLOW. This is a non-fatal error and is expected behavior in
that case. Don't handle this as an error, and do use the extracted data.

Fixes various cases of missing the last chunk of a page (pastie.org,
Google search results, and more).