MINOR: lua: properly process the contents of the content-length field

MINOR: lua: properly process the contents of the content-length field

The header's value was parsed with atoi() then compared against -1,
meaning that all the unparsable stuff returning zero was not considered
and that all multiples of 2^32 + 0xFFFFFFFF would continue to emit a
chunk.

Now instead we parse the value using a long long, only accept positive
values and consider all unparsable values as incorrect and switch to
either close or chunked encoding. This is more in line with what a
client (including haproxy's parser) would expect.

This may be backported as a cleanup to stable versions, though it's
really unlikely that Lua applications are facing side effects of this.