Disabled setting 'User-Agent' in browser environments by default

refs https://github.com/TryGhost/Toolbox/issues/348
refs https://github.com/TryGhost/Toolbox/issues/301
closes  #426
refs https://bugs.chromium.org/p/chromium/issues/detail?id=571722

- When Content API SDK is used in Chrome browser the requests are throwing "Refused to set unsafe header "User-Agent" errors. This is a bug in chrome due to historical reasons - the "User-Agent" header used to be on a "forbidden header names" list and still is in chrome.
- In case the client needs to send the User-Agent header in Firefox for example the SDK can be configured with `userAgent: true` parameter

Author: naz

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

@@ -1,6 +1,7 @@
 import axios from 'axios';
 import packageInfo from '../package.json';
 
+// @NOTE: this value is dynamically replaced based on browser/node environment
 const USER_AGENT_DEFAULT = true;
 
 const packageVersion = packageInfo.version;

packages/content-api/rollup.config.js

@@ -60,7 +60,8 @@ export default [
                 exclude: ['node_modules/**', '../../node_modules/**']
             }),
             replace({
-                'process.env.NODE_ENV': `"${process.env.NODE_ENV}"`
+                'process.env.NODE_ENV': `"${process.env.NODE_ENV}"`,
+                'USER_AGENT_DEFAULT = true': `USER_AGENT_DEFAULT = false`
             })
         ]
     },
@@ -97,7 +98,8 @@ export default [
                 exclude: ['node_modules/**', '../../node_modules/**']
             }),
             replace({
-                'process.env.NODE_ENV': `"${process.env.NODE_ENV}"`
+                'process.env.NODE_ENV': `"${process.env.NODE_ENV}"`,
+                'USER_AGENT_DEFAULT = true': `USER_AGENT_DEFAULT = false`
             }),
             terser()
         ]