preserve leading zeroes when exporting private key to hex/wif

otherwise keys starting with a zero would end up having a different address when imported somewhere else.

Tested against  the Bitcoin::Key library in bitcoin-ruby and electrum's import feature.

Not sure if this is the best way to fix it, since #int_to_hex is also used for other things that might be <64 chars - but at least it doesn't break the tests.

PS: I was able to reclaim the stuck funds by manually prepending "00" to the exported hex, before importing it into bitcoin-ruby.

PPS: Also thanks a lot for the library in the first place - I was really thrilled when I first saw it! :)