Incorrect X-Forwarded-For header

When X-Forwarded-For header appears multiple times in a request only the
last value is supplied by yaws_api:headers_x_forwarded_for/1.

RFC 2616 section 4.2 permits multiple headers with the same name as an
equivalent of one header with a comma-separated list of values.

http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2

Pound loadbalancer ( http://www.apsis.ch/pound ) repeats the header.

Change yaws to collect all values for this header and present them as a
single comma-separated list. Add a regression test to test/t2.