Initialize measurement api / fallback proxy in same way as analytics.js

[rdwallis]

When running analytics via the client, you always get a lot of extra data tied to each call.

ie. if you do a pagetrack call you can see if it came from mobile / desktop, if the user has flash enabled etc.

When using the measurement api you get the same if client analytics is not blocked.

But if client analytics is blocked then the measurement api will generate the user without setting any of the extra data. So the same pagetrack call won't include any of the extra information.

Mainly this is a concern when using a fallback proxy. Where you're making the same calls but events from users on the fallback are registering in a completely different way. So for example your data on what percentage of users are on mobile vs desktop will be incorrect.