Support requestOptions also being functions. This allows us to dynamically generate options at call-time for things like timestamp-sensitive authentication headers.
Added a test that makes sure requestOptions as functions behave in the expected way
Coverage increased (+0.1%) to 96.774% when pulling 5018469876e873dde21cb53e08ad3bd884584a75 on andreasrs:feature/headers-runtime into 823c67557844fd458d798af604392b50422ffe82 on popomore:master.