Lazily executed calculating headers in API framework

- 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

ghost/api-framework/lib/http.js

@@ -64,7 +64,6 @@ const http = (apiImpl) => {
             const result = await apiImpl(frame);
 
             debug(`External API request to ${frame.docName}.${frame.method}`);
-            const apiHeaders = await headers.get(result, apiImpl.headers, frame) || {};
 
             // CASE: api ctrl wants to handle the express response (e.g. streams)
             if (typeof result === 'function') {
@@ -82,6 +81,7 @@ const http = (apiImpl) => {
             res.status(statusCode);
 
             // CASE: generate headers based on the api ctrl configuration
+            const apiHeaders = await headers.get(result, apiImpl.headers, frame) || {};
             res.set(apiHeaders);
 
             const send = (format) => {