Fix bug with checking header variable

header is set outside haproxy_response, so it's always defined.  Check changed to
non-nil value, instead of using defined?.

This bug was introduced in 67db1dd.

Author: Ben Prew

check_haproxy.rb

@@ -146,11 +146,10 @@ def exit_code(code=0)
 header = nil
 
 haproxy_response(options).each do |line|
-
   if line =~ /^# /
     header = line[2..-1].split(',')
     next
-  elsif !defined? header
+  elsif !header
     puts "ERROR: CSV header is missing"
     exit UNKNOWN
   end