Reduce number of false negatives by comparing decoded message bodys

Message bodies encoded as quoted-printable are sometimes reformatted
(probably to conform to a 76 characters per line limit). This does not
change the actual (decoded) content, but confuses the diff algorithm if
the raw bodies are compared. With this patch, the bodies are first decoded
and then compared.

The patch only decodes non-multipart messages; the others are compared as before.