Incorrect usage of urlsafe-base64 leading to errors.

[SimonSchick]

Incorrect usage of urlsafe-base64 in jwt/decrypt.js:99:
https://github.com/cisco/node-jose/blob/master/lib/jwe/decrypt.js#L99

Assumes that the function has a second parameter for encoding, which is incorrect:
https://github.com/RGBboy/urlsafe-base64/blob/master/lib/urlsafe-base64.js#L50

This causes an error for me locally as it tries to JSON.parse the resulting Buffer.

SyntaxError: Unexpected token � in JSON at position 0
    at parse (<anonymous>)
    at JSON.parse (/Users/xxx/xxx/xxx/xxx/tests/_setup.js:20:20)
    at /Users/xxx/xxx/xxx/xxx/node_modules/node-jose/lib/jwe/decrypt.js:100:26
    at process._tickDomainCallback (internal/process/next_tick.js:129:7) { '0': <Buffer REDACTED ... > }

Proposed fix: Call toString on the buffer.