make OR filter work

[Setogit]

With this change, the following IMAP filter now works as follows;
Unseen AND size<500MB AND (FROM : tedj7890@gmail.com OR abc@example.com OR xyz@public.com OR 'Tetsuo Seto')

var FIVE_HUNDRED_MB = (50_1024_1024).toString()
searchFilter: ['UNSEEN', ['SMALLER',FIVE_HUNDRED_MB], 'OR',
['OR', ['FROM','tedj7890@gmail.com'],['FROM','abc@example.com']],
['OR', ['FROM','xyz@public.com'],['FROM','Tetsuo Seto']]
],

[mscdex]

Please add tests and possibly adjust the documentation to reflect this new behavior.