BUG/MINOR: htx: Transfer HTX_FL_EOM flag on success in htx_append_msg()

htx_append_msg() function copy all blocks from a source message to a
destination one. But it never take care to also transfer HTX_FL_EOM flag if
necessary on success. It is important because this function is used to copy
error messages during HTTP analysis.

It seems to be harmless because when an error is triggered the stream is
also closed and most of time a raw copy is performed instead of a
block-per-block copy. But this could lead to prematurely close the
connection at the end of the response.

This patch should be backported to all supported versions.