BUG/MINOR: hlua: _hlua_http_msg_delete incorrect behavior when offset is used

BUG/MINOR: hlua: _hlua_http_msg_delete incorrect behavior when offset is used

Calling the function with an offset when "offset + len" was superior or equal
to the targeted blk length caused 'v' value to be improperly set.
And because 'v' is directly provided to htx_replace_blk_value(), blk consistency was compromised.
(It seems that blk was overrunning in htx_replace_blk_value() due to
this and header data was overwritten in this case).

This patch adds the missing checks to make the function behave as
expected when offset is set and offset+len is greater or equals to the targeted blk length.
Some comments were added to the function as well.

It may be backported to 2.6 and 2.5