Fix Non-static method calls at CRM_Utils_Mail_Incoming class
On PHP7.1 class CRM_Utils_Mail_Incoming returns strict warnings due to inconsistency in function declarations
Strict warning: Non-static method CRM_Utils_Mail_Incoming::formatMailMultipart() should not be called statically w CRM_Utils_Mail_Incoming::formatMailMultipart()(linia 83 z /srv/staging/ocal.stg.civicrm.pl/public/sites/all/modules/civicrm/CRM/Utils/Mail/Incoming.php).