Support for repeated headers in responses in the Lua plugin

This is needed for WSAPI conformance, and important for having
good support of Set-Cookie.

Tested with: https://gist.github.com/cyx/8aeb704e325c747298ac

Should I add tests for this? if yes, where?

Relevant:

https://github.com/keplerproject/wsapi/blob/master/src/wsapi/common.lua#L170-L178
https://github.com/keplerproject/wsapi/blob/master/src/wsapi/response.lua#L79