class Mail::Body

def encoded(transfer_encoding = nil)

TODO: Validate that preamble and epilogue are valid for requested encoding
Calling this directly is not a good idea, but supported for compatibility
identiy encoding (i.e. no encoding).
Returns a body encoded using transfer_encoding. Multipart always uses an 
def encoded(transfer_encoding = nil)
  if multipart?
    self.sort_parts!
    encoded_parts = parts.map { |p| p.encoded }
    ([preamble] + encoded_parts).join(crlf_boundary) + end_boundary + epilogue.to_s
  else
    dec = Mail::Encodings.get_encoding(encoding)
    enc =
      if Utilities.blank?(transfer_encoding)
        dec
      else
        negotiate_best_encoding(transfer_encoding)
      end
    if dec.nil?
      # Cannot decode, so skip normalization
      raw_source
    else
      # Decode then encode to normalize and allow transforming 
      # from base64 to Q-P and vice versa
      decoded = dec.decode(raw_source)
      if defined?(Encoding) && charset && charset != "US-ASCII"
        decoded = decoded.encode(charset)
        decoded.force_encoding('BINARY') unless Encoding.find(charset).ascii_compatible?
      end
      enc.encode(decoded)
    end
  end
end