Fix warnings about uninitialized values:

Use of uninitialized value in uc at .../Text/vCard/Node.pm line 407.
Use of uninitialized value $t in uc at .../Text/vCard/Node.pm line 409.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Drop charset conversation in as_string for the moment as it's broken

Assume you import an UTF-8 and quoted-printable encoded vCard. During
export via as_string it gets double-encoded which breaks the exported
one.

Charset conversation is not trivial as there might be "not-map-able"
characters from original charset to the desired target charset. So
lets drop this feature for the moment and re-start later with a
clean approach.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>