analyzer: Don't strip extra characteres from body message

Although possible harmless, it might be better to leave the
message as close as the original was. So, from now, we avoid
removing leading characters at this stage.

Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>
analyzer: Decode headers with their own encoding. Fixes #13

Every header in an email can contain its own encoding information,
which works independently of the encoding of the message.
We know use that information instead of assuming the encoding
is the same for everything in a message

Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>
tests: Add tests for analyzer (mbox parser)

Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>