Updating 0-2jwt index.spec.js and index.js

[CoderInTheNorth-049]

index.js => As per the test case decoding output should be boolean and not object|false. Also, the solution provided for the assignment has a boolean output.

index.spec.js => Test cases for verifying JWT have a heading of decoding. (maybe you copied and pasted the test cases to save time but forgot to rename them).