Support reporting address modifiers in SPF records

When testing an email from a domain with a SPF record which contains ra=postmaster (e.g. v=spf1 mx ra=postmaster -all), the Sender Policy Framework test is marked as Invalid with the following message:

unknown modifier 'ra=postmaster'

ra along with rp and rr are however valid modifiers according to RFC6652 and should be considered as valid.

Example report: https://happydeliver.org/test/hadvvb5-4uzf63h-huynlld-x6dji