Fix a crash or text-mangling issue when an inbound message
uses a charset other than utf-8 for its text or html body,
and SendGrid's "post raw" inbound parse option is *not*
enabled.
Update docs to recommend "post raw" option.
Fixes#187
SendGrid does not always correctly provide the sent Message-ID header value
to a tracking webhook's smtp-id field, making it unreliable to use for Anymail's
`message_id`.
Instead, generate a UUID `message_id` for Anymail tracking, and pass it from
send to webhooks in SendGrid custom args as anymail_id.
Webhooks will fall back to smtp-id for compatibility with previously-sent
messages that didn't have an anymail_id custom arg.
Fixes#108
