HPCC-12127 Use m_content_length64 for Content-Length

Both m_content_length64 (int64) and m_content_length (int) are
used to store the length of an HTTP message content. When an
HTTP message content is very large, m_content_length may store
an incorrect value. ESP should use the m_content_length64 to
set the Content-Length header.

Signed-off-by: wangkx <kevin.wang@lexisnexis.com>