Improve error message for exp claim in payload

[excpt]

A better error message is displayed if a Time object is used for the exp
claim instead of an integer unix time stamp.

Fixes #148.

[mgwidmann]

This should have never been ported back to 1.5.5 as it is not backwards compatible since the payload object is not guaranteed in any other location to be a Hash object.

Passing in [] or any other object which does not respond to payload['exp'] crashes with:

TypeError: no implicit conversion of String into Integer

i.e. this crashes:

[1] pry(main)> token = JWT.encode ['my-data', {some: 'payload'}], nil, 'none'
TypeError: no implicit conversion of String into Integer
from (pry):2:in `[]'