BUG/MINOR: hlua: fixing hlua_http_msg_insert_data behavior

BUG/MINOR: hlua: fixing hlua_http_msg_insert_data behavior

hlua_http_msg_insert_data() function is called upon
HTTPMessage.insert() method from lua script.

This function did not work properly for multiple reasons:

  - An incorrect argument check was performed and prevented the user
  from providing optional offset argument.

  - Input and output variables were inverted
  and offset was not handled properly. The same bug
  was also fixed in hlua_http_msg_del_data(), see:
  'BUG/MINOR: hlua: fixing hlua_http_msg_del_data behavior'

The function now behaves as described in the documentation.

This could be backported to 2.6 and 2.5.