Fixes some issues with authentication against erchef

I brought privateEncrypt in from chef-golang to fix a problem with signing with SignPKCS1v15.

Also:
 - Including the right set of headers for erchef (as of 11.1.3-1).
 - Fixing a case problem with X-Ops-UserId.
 - Removes a trailing \n from the signed content, which erchef gets upset about.