Replace uses of `as_string` from the email lib.

The `as_string` function from `email.message.Message` assumes that it's being
used to encode a message for, well, email transmission -- and as a result, it
changes lines beginning with "From " into lines beginning with ">From ". Given
that we're using that module for multipart uploads, this behavior is surely
not what we want.

We basically inline the body of `as_string`, setting `mangle_from_` to
`False`.