don't use localhost as the HELO on outgoing SMTP connections

It can be configured using `dns.helo_hostname` or will be set to `dns.smtp_server_hostname`

Related to #19

lib/postal/smtp_sender.rb

@@ -57,7 +57,7 @@ module Postal
             else
               # Nothing
             end
-            smtp_client.start(@source_ip_address ? @source_ip_address.hostname : "localhost")
+            smtp_client.start(@source_ip_address ? @source_ip_address.hostname : self.class.default_helo_hostname)
             log "Connected to #{@remote_ip}:#{port} (#{hostname})"
           rescue => e
             log "Cannot connect to #{@remote_ip}:#{port} (#{hostname}) (#{e.class}: #{e.message})"
@@ -244,5 +244,9 @@ module Postal
       end
     end
 
+    def self.default_helo_hostname
+      Postal.config.dns.helo_hostname || Postal.config.dns.smtp_server_hostname || "localhost"
+    end
+
   end
 end