Provide original function name in callback

[ewilde]

The callback currently contains the following meta-data from the original invocation:

Header	Description
X-Call-Id	The original function call's tracing UUID
X-Duration-Seconds	Time taken in seconds to execute the original function call
X-Function-Status	HTTP status code returned by the original function call

I am proposing that we add the original function name as well

Header	Description
X-Function-Name	The original function name that was invoked

Use case

With this extra piece of meta-data people can design their own back-off / retry / eventual consistency systems, without the need to modify any core OpenFaaS components

Consider this design now enabled by the extra field

[alexellis]

I am fine with this.