Added support for UseDefaultCredentials

This PR adds the support for UseDefaultCredentials that we discussed in #452. I placed it in IRestRequest rather than in the IRestClient, which makes it possible to select it on a request-by-request basis whether the flag should be on or off.

I've tried to keep it as non-noisy as possible. Made one minor spelcheck & a minor rearrangement of some lines, but my changes shouldn't be more invasive than so. Please, investigate the changes and let me know if you have some comments. Otherwise, looking forward to get this integrated asap. :smile: 

Cheers,
Per