BUG/MEDIUM: htx: Don't reset flags when source must be preserved during transfer

BUG/MEDIUM: htx: Don't reset flags when source must be preserved during transfer

When htx_xfer() is called with HTX_XFER_KEEP_SRC_BLKS flag, to preserve
blocks from the source HTX message, the source flags must not be reset at
the end of the transfer.

This bug can be triggered with L7-retries are configured with small
buffers. In that case, the HTX_FL_EOM flag is removed from the request
fowarded to the backend multiplexer.

This patch must be backported to 3.4.