Make `AnymailInboundMessage.text`, `.html` and `.get_content_text()`
usually do the right thing for non-UTF-8 messages/attachments. Fixes
an incorrect UnicodeDecodeError when receiving an (e.g.,) ISO-8859-1
encoded message, and improves handling for inbound messages that were
not properly encoded by the sender.
* Decode using the message's (or attachments's) declared charset
by default (rather than always defaulting to 'utf-8'; you can
still override with `get_content_text(charset=...)`
* Add `errors` param to `get_content_text()`, defaulting to 'replace'.
Mis-encoded messages will now use the Unicode replacement character
rather than raising errors. (Use `get_content_text(errors='strict')`
for the previous behavior.)
