Add header "Proxy-Authenticate" into 407 response.
Add header "Proxy-Authenticate" into 407 response.

This fixes https://github.com/abhinavsingh/proxy.py/issues/49

abhinavsingh/proxy.py,abhinavsingh/proxy.py,abhinavsingh/proxy.py,abhinavsingh/proxy.py,abhinavsingh/proxy.py,abhinavsingh/proxy.py