Uppercased Content-Version header use

refs https://github.com/TryGhost/Toolbox/issues/227

- The convention across the codebase is to define headers with capitalized first letters. This change does not affect the output though as all headers are served lowercased anyway.
- Might be a good idea to make all headers lowercased one day to match the casing with the outputs

core/server/api/shared/http.js

@@ -83,7 +83,7 @@ const http = (apiImpl) => {
 
             // CASE: generate headers based on the api ctrl configuration
             if (req && req.headers && req.headers['accept-version'] && res.locals) {
-                headers['content-version'] = `v${res.locals.safeVersion}`;
+                headers['Content-Version'] = `v${res.locals.safeVersion}`;
             }
             res.set(headers);