[jinterface] added OtpMbox.hash()

[Vlad Dumitrescu]

Hi,

The OtpMbox class was missing the hash() method while overriding
equals(). This can cause problems when when using jinterface in a
larger Java application, for which there are currently no tests in
OTP. The fix is straightforward, if a test is required please let me
know.

git fetch git://github.com/vladdu/otp.git otpmbox_hash

best regards,
Vlad