Segment UserID loss

[xtreding]

UserId field exists in Segment /p (page) requests, but EN doesn't capture this field. It might happen because analytics.js enrich requests with additional parameters (like userId) after middleware.

Step to reproduce: go to the track demo page, send page event, and look through Segment and EN requests in dev tools
Expected result: EN request contains userId in src_payload.obj.userId
Actual result: EN request doesn't contain userId