remove duplicated "X-Forwarded-For" from ENV_HEADERS
remove duplicated "X-Forwarded-For" from ENV_HEADERS

krissman/httpbin,kennethreitz/httpbin,ewdurbin/httpbin,Runscope/httpbin,kennethreitz/httpbin,postmanlabs/httpbin,postmanlabs/httpbin,ewdurbin/httpbin,krissman/httpbin,Runscope/httpbin