- if the API controller endpoint is a function, we early return as we
expect the function to handle the response but we still ended up
calculating the headers beforehand, only to be thrown away
- this commit moves the header fetching code down in the flow so it's
only executed when needed
- this doesn't really have a big effect for us because 99% of our
controllers follow the object pattern
refs https://github.com/TryGhost/Toolbox/issues/363
- this API framework is standalone and should be pulled out into a
separate package so we can define its boundaries more clearly, and
promote better testing of smaller parts
