Fix sendgrid cc and bcc

[andrewwhipple]

Resolves a few interrelated issues with adding CC and BCC addresses to messages using the sendgrid V3 adapter:

1. When using add_bcc(), messages are never delivered to the bcc address
2. When using add_cc(), sendgrid returns a 400 Bad Response error with the following: Each email address in the personalization block should be unique between to, cc, and bcc. We found the first duplicate instance of [email@example.com] in the personalizations.1.bcc field

The two issues above come from a tiny typo, where the section for handling bccs in the message_constructor leads to pulling emails from the cc list instead of the bcc list (line 124)

if self.bcc_list:
    list_bcc = [Bcc(email) for email in self.cc_list]
    message.bcc = list_bcc

Fixing those issues then leads to:

3. When using add_cc() or add_bcc(), sendgrid returns a 400 Bad Request error with the following: The to array is required for all personalization objects, and must have at least one email object with a valid email address.

This is caused by using the add_to function with is_multiple=True, which causes sendgrid to create a new Personalization for each recipient of the email message. In that case then, the bcc and cc fields need to be added on to each of the personalization objects, not the message object, so this reworks the logic to do that (semi-related StackOverflow example.)

Also for now chose to keep the is_multiple=True behavior and work around it rather than another approach, to keep things as consistent as possible to how it works today (just with bcc and cc working); there certainly may be a more elegant sendgrid-approved approach! Also added a quick little cc/bcc addendum to the existing test for message_constructor

(For SpotOn internal users, this is for jira ticket CSVC-520, discovered while working on CSVC-381 )

[mspiegel31]

according to the the source for MassEmail (linked here), self.cc_list and self.bcc_list are always initialized as empty lists. I think you can just simplify this to:

for cc_email in self.cc_list:
    personalization.add_cc(...)

for bcc_email in self.bcc_list:
    personalization.add_bcc(...)

[andrewwhipple]

Ooo nice catch 👍