Enabled setting custom 'User-Agent' header

refs https://github.com/TryGhost/Toolbox/issues/348
refs https://github.com/TryGhost/Toolbox/issues/301

- Allows for a full control over the 'User-Agent' header. Enables customizing the header in whatever way the client needs (for example prepending a value when there's a wrapping library using the SDK)

Author: naz

packages/content-api/lib/content-api.js

@@ -53,7 +53,7 @@ const defaultMakeRequest = ({url, method, params, headers}) => {
  * @param {String} options.key
  * @param {String} [options.ghostPath]
  * @param {String|Boolean} options.version - a version string like v3, v4, v5 or boolean value identifying presence of Accept-Version header
- * @param {Boolean} [options.userAgent] - flag controlling if the 'User-Agent' header should be sent with a request
+ * @param {String|Boolean} [options.userAgent] - value controlling the 'User-Agent' header should be sent with a request
  * @param {Function} [options.makeRequest]
  */
 export default function GhostContentAPI({url, key, version, userAgent, ghostPath = 'ghost', makeRequest = defaultMakeRequest}) {
@@ -152,7 +152,11 @@ export default function GhostContentAPI({url, key, version, userAgent, ghostPath
         } : {};
 
         if (userAgent) {
-            headers['User-Agent'] = `GhostContentSDK/${packageVersion}`;
+            if (typeof userAgent === 'boolean') {
+                headers['User-Agent'] = `GhostContentSDK/${packageVersion}`;
+            } else {
+                headers['User-Agent'] = userAgent;
+            }
         }
 
         if (acceptVersionHeader) {

packages/content-api/test/content-api-test/content-api.test.js

@@ -237,5 +237,28 @@ describe('GhostContentApi', function () {
             should.equal(makeRequestStub.args[0][0].headers['Accept-Version'], 'v5.0');
             should.equal(makeRequestStub.args[0][0].headers['User-Agent'], undefined);
         });
+
+        it('Sets a custom User-Agent header', async function () {
+            const makeRequestStub = sinon.stub().returns(Promise.resolve({
+                data: {
+                    settings: {}
+                }
+            }));
+
+            const api = new GhostContentApi({
+                version: 'canary',
+                url: `http://ghost.local`,
+                key: '0123456789abcdef0123456789',
+                makeRequest: makeRequestStub,
+                userAgent: 'I_LOVE_CUSTOM_THINGS'
+            });
+
+            await api.settings.browse();
+
+            makeRequestStub.calledOnce.should.be.true();
+            should.equal(makeRequestStub.args[0][0].url, 'http://ghost.local/ghost/api/canary/content/settings/');
+            should.equal(makeRequestStub.args[0][0].headers['Accept-Version'], 'v5.0');
+            should.equal(makeRequestStub.args[0][0].headers['User-Agent'], 'I_LOVE_CUSTOM_THINGS');
+        });
     });
 });