Allow 407 status code

Hi, 

This patch allows nginx to return a status code of 407 'Proxy Authentication Required' (which is disabled by default in nginx). With lua, we can manage the Proxy-Authenticate header, and make openresty behave as a real proxy with authorization.

It's my first patch to this project so if there's something wrong with it, please tell, and I'll try to fix it accordingly.
The patch is for 1.4.2, 1.4.3 and I also added the patch for 1.5.6 which  I guess is the one for next releases although I didn't know how to try it. I looked at 1.5.6 code and it should work also, but as I said, it's untested.

Regards and thanks for this awesome project!