BUG/MINOR: hlua: hlua_channel_insert_data() behavior conflicts with documentation

BUG/MINOR: hlua: hlua_channel_insert_data() behavior conflicts with documentation

Channel.insert(channel, string, [,offset]):

When no offset is provided, hlua_channel_insert_data() inserts
string at the end of incoming data.

This behavior conflicts with the documentation that explicitly says
that the default behavior is to insert the string in front of incoming data.

This patch fixes hlua_channel_insert_data() behavior so that it fully
complies with the documentation.

Thanks to Smackd0wn for noticing it.

This could be backported to 2.6 and 2.5