(proxy) set unset description on X-Forwarded-For. closes...

(proxy) set unset description on X-Forwarded-For. closes https://github.com/opnsense/core/issues/790

src/opnsense/mvc/app/models/OPNsense/Proxy/Proxy.xml

@@ -42,6 +42,7 @@
             <forwardedForHandling type="OptionField">
                 <default>on</default>
                 <Required>N</Required>
+                <BlankDesc>Default</BlankDesc>
                 <OptionValues>
                     <on>Append client's IP (on)</on>
                     <off>Set forward header to unknown (off)</off>