MEDIUM: hlua: Add function to change the body length of an HTTP Message

MEDIUM: hlua: Add function to change the body length of an HTTP Message

There was no function for a lua filter to change the body length of an HTTP
Message. But it is mandatory to be able to alter the message payload. It is
not possible update to directly update the message headers because the
internal state of the message must also be updated accordingly.

It is the purpose of HTTPMessage.set_body_len() function. The new body
length myst be passed as argument. If it is an integer, the right
"Content-Length" header is set. If the "chunked" string is used, it forces
the message to be chunked-encoded and in that case the "Transfer-Encoding"
header.

This patch should fix the issue #2837. It could be backported as far as 2.6.