Added test cases for all scenarios in request handler.

*According to [coveralls.io](https://coveralls.io/r/Codesleuth/esendex-node), the code has 100% coverage.*

This work was done to increase the coverage for the requesthandler module, but I know that the callbacks for errors in the higher level modules are not tested. This is probably a side effect from something to do with the coverage client being used ([istanbul](https://github.com/gotwarlost/istanbul)) so more work has yet to be done to resolve that.