Adding support for Salesforce specific headers on REST requests

[colinfindlay-nz]

We have a need to pass through a few Salesforce headers when making requests.
Namely some of the conditional headers like If-Modified-Since as well as Sforce-Auto-Assign
We have a PR on our fork at vacasaoss#8

It would be great to get any feedback from @ChuckJonas before we finalise the code, as we'd love to submit this back to the community.