Incorrect X-Forwarded-For header

When X-Forwarded-For header appears multiple times in request only
the last value is present in yaws_api:headers_x_forwarded_for/1

The RFC 2616, section 4.2 permits multiple headers with same name
as an equivalent of one header with 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.