Support multiple encodings

[hugorodgerbrown]

Emails can have different encodings set on different fields, and these may well be different from the encoding used for the HTTP request - e.g. and email sent from Outlook running on Windows may have the 'to' and 'subject' fields encoded in 'windows-1252', which is then forwarded on via the backend provider in an HTTP request encoding with UTF-8. Different backends will deal with this in their own way, but as an example, SendGrid sends a 'charsets' value in the request, which is a JSON-formatted string that contains the specific encoding for each field. This can then be used by the backend to decode the values sent.