* Set to field when using merge_data
The `to` field is required even if providing recipient addresses in x-smtpapi. See https://sendgrid.com/docs/API_Reference/Web_API/mail.html#-send.
* Check data['to'] contains expected emails
* Add space for toname check
* Make `to` expected data contain email only
* Create generic TestBackend that simply collects
send parameters
* Change BackendSettingsTests to TestBackend,
and add some missing cases
* Add UnsupportedFeatureTests
* Replace repetitive per-backend SEND_DEFAULTS
test cases with single (and more comprehensive)
SendDefaultsTests
* message.template_id to use ESP stored templates
* message.merge_data and merge_global_data
to supply per-recipient/global merge variables
(with or without an ESP stored template)
* When using per-recipient merge_data, tell ESP to use
batch send: individual message per "to" address.
(Mailgun does this automatically; SendGrid requires
using a different "to" field; Mandrill requires
`preserve_recipients=False`; Postmark doesn't
support *this type* of batch sending with merge data.)
* Allow message.from_email=None (must be set after
init) and message.subject=None to suppress those
fields in API calls (for ESPs that allow "From" and
"Subject" in their template definitions).
Mailgun:
* Emulate merge_global_data by copying to
recipient-variables for each recipient.
SendGrid:
* Add delimiters to merge field names via
esp_extra['merge_field_format'] or
ANYMAIL_SENDGRID_MERGE_FIELD_FORMAT setting.
Mandrill:
* Remove Djrill versions of these features;
update migration notes.
Closes#5.
Previously, setting esp_extra['x-smtpapi']['filters']
would override the entire filters setting, potentially
undoing other Anymail options that use SendGrid
filters (like track_opens).
Now, 'filters' is special-cased, and merged with
any other Anymail filter options.
(We don't do a fully deep merge, because otherwise
there would be no way to use esp_extra to *clear*
Anymail settings.)
* Add smtp-id in unique_args (metadata), to ensure
it shows up in click and open events.
* Add SENDGRID_GENERATE_MESSAGE_ID setting,
default True, to control auto-Message-ID behavior.
* Document it.
