update getSessionKey

[halilcakar]

Hi this PR is about updating getSessionKey

From npm log's i saw that creating a new Buffer is deprecating, instead we should use, Buffer.alloc(), Buffer.from() functions. I updated those and also,

when we use unserialize() function it was always giving us a part of sessionID, i updated that to

line -> 25
decoded.toString() + decoder.final().toString()
now give's you all ID,

Please give it a try :)

[halilcakar]

I think this will also resolve #11 problem too But i didn't get why Merging is blocked ❓