Do not modify a Dictionary while iterating over KeyValuePairs
Do not modify a Dictionary while iterating over KeyValuePairs

Fixes issue #201

mganss/MailKit,nachocove/MailKit,eino-makitalo/MailKit,jamie-dainton/MailKit,sstraus/MailKit,matt2003/MailKit,dsbenghe/MailKit,jstedfast/MailKit