charset UTF-8, not utf-8

They are equivalent, but David Golden was giving me that
sideeye...
test for (and fix) breaking of binary attachments

1.  Since we now futz with encoding/decoding more often (and more
    correctly) make sure it doesn't affect binary files.

2.  Realize that even without the encoding, we were breaking binary
    files by adding CRLF even to non-text.
switch subtest-like code to use a subtest
preliminary changelog for encoding changes
backpedal on a feature being "probably" available "soon"
document the changes in encoding behavior
update changelog
v3.000000

          [BREAKING CHANGES]
        - content read from disk (templates, for example) is now decoded
          as UTF-8 if and only if the content type for which it is being used
          is text/*
        - for more information on this change, consult the "ENCODING" section
          of the documentation for Email::MIME::Kit

          [BUG FIXES]
        - non-text parts are no longer padded with CRLFs