- Change tests to use know correct signature

The issue with the current test is that if there is an error in the way that signatures are being generated, the test would still pass since the module and test are generating the signature using the same code.

This is because the test is generating a signature in the same way the the module is so they will always match.

If we test using the know correct signature then if signature creation code starts generating an incorrect signature then the tests will fail
Merge branch 'test-using-known-signature' of https://github.com/tekay/twilio-node into use-know-signatures